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	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * fs/dcache.c
  *
  * Complete reimplementation
  * (C) 1997 Thomas Schoebel-Theuer,
  * with heavy changes by Linus Torvalds
  */
 
 /*
  * Notes on the allocation strategy:
  *
  * The dcache is a master of the icache - whenever a dcache entry
  * exists, the inode will always exist. "iput()" is done either when
  * the dcache entry is deleted or garbage collected.
  */
 
 #include <linux/ratelimit.h>
 #include <linux/string.h>
 #include <linux/mm.h>
 #include <linux/fs.h>
 #include <linux/fscrypt.h>
 #include <linux/fsnotify.h>
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/hash.h>
 #include <linux/cache.h>
 #include <linux/export.h>
 #include <linux/security.h>
 #include <linux/seqlock.h>
 #include <linux/memblock.h>
 #include <linux/bit_spinlock.h>
 #include <linux/rculist_bl.h>
 #include <linux/list_lru.h>
 #include "internal.h"
 #include "mount.h"
 
 /*
  * Usage:
  * dcache->d_inode->i_lock protects:
  *   - i_dentry, d_u.d_alias, d_inode of aliases
  * dcache_hash_bucket lock protects:
  *   - the dcache hash table
  * s_roots bl list spinlock protects:
  *   - the s_roots list (see __d_drop)
  * dentry->d_sb->s_dentry_lru_lock protects:
  *   - the dcache lru lists and counters
  * d_lock protects:
  *   - d_flags
  *   - d_name
  *   - d_lru
  *   - d_count
  *   - d_unhashed()
  *   - d_parent and d_subdirs
  *   - childrens' d_child and d_parent
  *   - d_u.d_alias, d_inode
  *
  * Ordering:
  * dentry->d_inode->i_lock
  *   dentry->d_lock
  *     dentry->d_sb->s_dentry_lru_lock
  *     dcache_hash_bucket lock
  *     s_roots lock
  *
  * If there is an ancestor relationship:
  * dentry->d_parent->...->d_parent->d_lock
  *   ...
  *     dentry->d_parent->d_lock
  *       dentry->d_lock
  *
  * If no ancestor relationship:
  * arbitrary, since it's serialized on rename_lock
  */
 int sysctl_vfs_cache_pressure __read_mostly = 100;
 EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);
 
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);
 
 EXPORT_SYMBOL(rename_lock);
 
 static struct kmem_cache *dentry_cache __read_mostly;
 
 const struct qstr empty_name = QSTR_INIT("", 0);
 EXPORT_SYMBOL(empty_name);
 const struct qstr slash_name = QSTR_INIT("/", 1);
 EXPORT_SYMBOL(slash_name);
 const struct qstr dotdot_name = QSTR_INIT("..", 2);
 EXPORT_SYMBOL(dotdot_name);
 
 /*
  * This is the single most critical data structure when it comes
  * to the dcache: the hashtable for lookups. Somebody should try
  * to make this good - I've just made it work.
  *
  * This hash-function tries to avoid losing too many bits of hash
  * information, yet avoid using a prime hash-size or similar.
  */
 
 static unsigned int d_hash_shift __read_mostly;
 
 static struct hlist_bl_head *dentry_hashtable __read_mostly;
 
 static inline struct hlist_bl_head *d_hash(unsigned int hash)
 {
     return dentry_hashtable + (hash >> d_hash_shift);
 }
 
 #define IN_LOOKUP_SHIFT 10
 static struct hlist_bl_head in_lookup_hashtable[1 << IN_LOOKUP_SHIFT];
 
 static inline struct hlist_bl_head *in_lookup_hash(const struct dentry *parent,
                     unsigned int hash)
 {
     hash += (unsigned long) parent / L1_CACHE_BYTES;
     return in_lookup_hashtable + hash_32(hash, IN_LOOKUP_SHIFT);
 }
 
 struct dentry_stat_t {
     long nr_dentry;
     long nr_unused;
     long age_limit;     /* age in seconds */
     long want_pages;    /* pages requested by system */
     long nr_negative;   /* # of unused negative dentries */
     long dummy;     /* Reserved for future use */
 };
 
 static DEFINE_PER_CPU(long, nr_dentry);
 static DEFINE_PER_CPU(long, nr_dentry_unused);
 static DEFINE_PER_CPU(long, nr_dentry_negative);
 
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
 /* Statistics gathering. */
 static struct dentry_stat_t dentry_stat = {
     .age_limit = 45,
 };
 
 /*
  * Here we resort to our own counters instead of using generic per-cpu counters
  * for consistency with what the vfs inode code does. We are expected to harvest
  * better code and performance by having our own specialized counters.
  *
  * Please note that the loop is done over all possible CPUs, not over all online
  * CPUs. The reason for this is that we don't want to play games with CPUs going
  * on and off. If one of them goes off, we will just keep their counters.
  *
  * glommer: See cffbc8a for details, and if you ever intend to change this,
  * please update all vfs counters to match.
  */
 static long get_nr_dentry(void)
 {
     int i;
     long sum = 0;
     for_each_possible_cpu(i)
         sum += per_cpu(nr_dentry, i);
     return sum < 0 ? 0 : sum;
 }
 
 static long get_nr_dentry_unused(void)
 {
     int i;
     long sum = 0;
     for_each_possible_cpu(i)
         sum += per_cpu(nr_dentry_unused, i);
     return sum < 0 ? 0 : sum;
 }
 
 static long get_nr_dentry_negative(void)
 {
     int i;
     long sum = 0;
 
     for_each_possible_cpu(i)
         sum += per_cpu(nr_dentry_negative, i);
     return sum < 0 ? 0 : sum;
 }
 
 static int proc_nr_dentry(struct ctl_table *table, int write, void *buffer,
               size_t *lenp, loff_t *ppos)
 {
     dentry_stat.nr_dentry = get_nr_dentry();
     dentry_stat.nr_unused = get_nr_dentry_unused();
     dentry_stat.nr_negative = get_nr_dentry_negative();
     return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 }
 
 static struct ctl_table fs_dcache_sysctls[] = {
     {
         .procname   = "dentry-state",
         .data       = &dentry_stat,
         .maxlen     = 6*sizeof(long),
         .mode       = 0444,
         .proc_handler   = proc_nr_dentry,
     },
     { }
 };
 
 static int __init init_fs_dcache_sysctls(void)
 {
     register_sysctl_init("fs", fs_dcache_sysctls);
     return 0;
 }
 fs_initcall(init_fs_dcache_sysctls);
 #endif
 
 /*
  * Compare 2 name strings, return 0 if they match, otherwise non-zero.
  * The strings are both count bytes long, and count is non-zero.
  */
 #ifdef CONFIG_DCACHE_WORD_ACCESS
 
 #include <asm/word-at-a-time.h>
 /*
  * NOTE! 'cs' and 'scount' come from a dentry, so it has a
  * aligned allocation for this particular component. We don't
  * strictly need the load_unaligned_zeropad() safety, but it
  * doesn't hurt either.
  *
  * In contrast, 'ct' and 'tcount' can be from a pathname, and do
  * need the careful unaligned handling.
  */
 static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
 {
     unsigned long a,b,mask;
 
     for (;;) {
         a = read_word_at_a_time(cs);
         b = load_unaligned_zeropad(ct);
         if (tcount < sizeof(unsigned long))
             break;
         if (unlikely(a != b))
             return 1;
         cs += sizeof(unsigned long);
         ct += sizeof(unsigned long);
         tcount -= sizeof(unsigned long);
         if (!tcount)
             return 0;
     }
     mask = bytemask_from_count(tcount);
     return unlikely(!!((a ^ b) & mask));
 }
 
 #else
 
 static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
 {
     do {
         if (*cs != *ct)
             return 1;
         cs++;
         ct++;
         tcount--;
     } while (tcount);
     return 0;
 }
 
 #endif
 
 static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
 {
     /*
      * Be careful about RCU walk racing with rename:
      * use 'READ_ONCE' to fetch the name pointer.
      *
      * NOTE! Even if a rename will mean that the length
      * was not loaded atomically, we don't care. The
      * RCU walk will check the sequence count eventually,
      * and catch it. And we won't overrun the buffer,
      * because we're reading the name pointer atomically,
      * and a dentry name is guaranteed to be properly
      * terminated with a NUL byte.
      *
      * End result: even if 'len' is wrong, we'll exit
      * early because the data cannot match (there can
      * be no NUL in the ct/tcount data)
      */
     const unsigned char *cs = READ_ONCE(dentry->d_name.name);
 
     return dentry_string_cmp(cs, ct, tcount);
 }
 
 struct external_name {
     union {
         atomic_t count;
         struct rcu_head head;
     } u;
     unsigned char name[];
 };
 
 static inline struct external_name *external_name(struct dentry *dentry)
 {
     return container_of(dentry->d_name.name, struct external_name, name[0]);
 }
 
 static void __d_free(struct rcu_head *head)
 {
     struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
 
     kmem_cache_free(dentry_cache, dentry); 
 }
 
 static void __d_free_external(struct rcu_head *head)
 {
     struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
     kfree(external_name(dentry));
     kmem_cache_free(dentry_cache, dentry);
 }
 
 static inline int dname_external(const struct dentry *dentry)
 {
     return dentry->d_name.name != dentry->d_iname;
 }
 
 void take_dentry_name_snapshot(struct name_snapshot *name, struct dentry *dentry)
 {
     spin_lock(&dentry->d_lock);
     name->name = dentry->d_name;
     if (unlikely(dname_external(dentry))) {
         atomic_inc(&external_name(dentry)->u.count);
     } else {
         memcpy(name->inline_name, dentry->d_iname,
                dentry->d_name.len + 1);
         name->name.name = name->inline_name;
     }
     spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(take_dentry_name_snapshot);
 
 void release_dentry_name_snapshot(struct name_snapshot *name)
 {
     if (unlikely(name->name.name != name->inline_name)) {
         struct external_name *p;
         p = container_of(name->name.name, struct external_name, name[0]);
         if (unlikely(atomic_dec_and_test(&p->u.count)))
             kfree_rcu(p, u.head);
     }
 }
 EXPORT_SYMBOL(release_dentry_name_snapshot);
 
 static inline void __d_set_inode_and_type(struct dentry *dentry,
                       struct inode *inode,
                       unsigned type_flags)
 {
     unsigned flags;
 
     dentry->d_inode = inode;
     flags = READ_ONCE(dentry->d_flags);
     flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
     flags |= type_flags;
     smp_store_release(&dentry->d_flags, flags);
 }
 
 static inline void __d_clear_type_and_inode(struct dentry *dentry)
 {
     unsigned flags = READ_ONCE(dentry->d_flags);
 
     flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
     WRITE_ONCE(dentry->d_flags, flags);
     dentry->d_inode = NULL;
     if (dentry->d_flags & DCACHE_LRU_LIST)
         this_cpu_inc(nr_dentry_negative);
 }
 
 static void dentry_free(struct dentry *dentry)
 {
     WARN_ON(!hlist_unhashed(&dentry->d_u.d_alias));
     if (unlikely(dname_external(dentry))) {
         struct external_name *p = external_name(dentry);
         if (likely(atomic_dec_and_test(&p->u.count))) {
             call_rcu(&dentry->d_u.d_rcu, __d_free_external);
             return;
         }
     }
     /* if dentry was never visible to RCU, immediate free is OK */
     if (dentry->d_flags & DCACHE_NORCU)
         __d_free(&dentry->d_u.d_rcu);
     else
         call_rcu(&dentry->d_u.d_rcu, __d_free);
 }
 
 /*
  * Release the dentry's inode, using the filesystem
  * d_iput() operation if defined.
  */
 static void dentry_unlink_inode(struct dentry * dentry)
     __releases(dentry->d_lock)
     __releases(dentry->d_inode->i_lock)
 {
     struct inode *inode = dentry->d_inode;
 
     raw_write_seqcount_begin(&dentry->d_seq);
     __d_clear_type_and_inode(dentry);
     hlist_del_init(&dentry->d_u.d_alias);
     raw_write_seqcount_end(&dentry->d_seq);
     spin_unlock(&dentry->d_lock);
     spin_unlock(&inode->i_lock);
     if (!inode->i_nlink)
         fsnotify_inoderemove(inode);
     if (dentry->d_op && dentry->d_op->d_iput)
         dentry->d_op->d_iput(dentry, inode);
     else
         iput(inode);
 }
 
 /*
  * The DCACHE_LRU_LIST bit is set whenever the 'd_lru' entry
  * is in use - which includes both the "real" per-superblock
  * LRU list _and_ the DCACHE_SHRINK_LIST use.
  *
  * The DCACHE_SHRINK_LIST bit is set whenever the dentry is
  * on the shrink list (ie not on the superblock LRU list).
  *
  * The per-cpu "nr_dentry_unused" counters are updated with
  * the DCACHE_LRU_LIST bit.
  *
  * The per-cpu "nr_dentry_negative" counters are only updated
  * when deleted from or added to the per-superblock LRU list, not
  * from/to the shrink list. That is to avoid an unneeded dec/inc
  * pair when moving from LRU to shrink list in select_collect().
  *
  * These helper functions make sure we always follow the
  * rules. d_lock must be held by the caller.
  */
 #define D_FLAG_VERIFY(dentry,x) WARN_ON_ONCE(((dentry)->d_flags & (DCACHE_LRU_LIST | DCACHE_SHRINK_LIST)) != (x))
 static void d_lru_add(struct dentry *dentry)
 {
     D_FLAG_VERIFY(dentry, 0);
     dentry->d_flags |= DCACHE_LRU_LIST;
     this_cpu_inc(nr_dentry_unused);
     if (d_is_negative(dentry))
         this_cpu_inc(nr_dentry_negative);
     WARN_ON_ONCE(!list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
 static void d_lru_del(struct dentry *dentry)
 {
     D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
     dentry->d_flags &= ~DCACHE_LRU_LIST;
     this_cpu_dec(nr_dentry_unused);
     if (d_is_negative(dentry))
         this_cpu_dec(nr_dentry_negative);
     WARN_ON_ONCE(!list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
 static void d_shrink_del(struct dentry *dentry)
 {
     D_FLAG_VERIFY(dentry, DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
     list_del_init(&dentry->d_lru);
     dentry->d_flags &= ~(DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
     this_cpu_dec(nr_dentry_unused);
 }
 
 static void d_shrink_add(struct dentry *dentry, struct list_head *list)
 {
     D_FLAG_VERIFY(dentry, 0);
     list_add(&dentry->d_lru, list);
     dentry->d_flags |= DCACHE_SHRINK_LIST | DCACHE_LRU_LIST;
     this_cpu_inc(nr_dentry_unused);
 }
 
 /*
  * These can only be called under the global LRU lock, ie during the
  * callback for freeing the LRU list. "isolate" removes it from the
  * LRU lists entirely, while shrink_move moves it to the indicated
  * private list.
  */
 static void d_lru_isolate(struct list_lru_one *lru, struct dentry *dentry)
 {
     D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
     dentry->d_flags &= ~DCACHE_LRU_LIST;
     this_cpu_dec(nr_dentry_unused);
     if (d_is_negative(dentry))
         this_cpu_dec(nr_dentry_negative);
     list_lru_isolate(lru, &dentry->d_lru);
 }
 
 static void d_lru_shrink_move(struct list_lru_one *lru, struct dentry *dentry,
                   struct list_head *list)
 {
     D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
     dentry->d_flags |= DCACHE_SHRINK_LIST;
     if (d_is_negative(dentry))
         this_cpu_dec(nr_dentry_negative);
     list_lru_isolate_move(lru, &dentry->d_lru, list);
 }
 
 static void ___d_drop(struct dentry *dentry)
 {
     struct hlist_bl_head *b;
     /*
      * Hashed dentries are normally on the dentry hashtable,
      * with the exception of those newly allocated by
      * d_obtain_root, which are always IS_ROOT:
      */
     if (unlikely(IS_ROOT(dentry)))
         b = &dentry->d_sb->s_roots;
     else
         b = d_hash(dentry->d_name.hash);
 
     hlist_bl_lock(b);
     __hlist_bl_del(&dentry->d_hash);
     hlist_bl_unlock(b);
 }
 
 void __d_drop(struct dentry *dentry)
 {
     if (!d_unhashed(dentry)) {
         ___d_drop(dentry);
         dentry->d_hash.pprev = NULL;
         write_seqcount_invalidate(&dentry->d_seq);
     }
 }
 EXPORT_SYMBOL(__d_drop);
 
 /**
  * d_drop - drop a dentry
  * @dentry: dentry to drop
  *
  * d_drop() unhashes the entry from the parent dentry hashes, so that it won't
  * be found through a VFS lookup any more. Note that this is different from
  * deleting the dentry - d_delete will try to mark the dentry negative if
  * possible, giving a successful _negative_ lookup, while d_drop will
  * just make the cache lookup fail.
  *
  * d_drop() is used mainly for stuff that wants to invalidate a dentry for some
  * reason (NFS timeouts or autofs deletes).
  *
  * __d_drop requires dentry->d_lock
  *
  * ___d_drop doesn't mark dentry as "unhashed"
  * (dentry->d_hash.pprev will be LIST_POISON2, not NULL).
  */
 void d_drop(struct dentry *dentry)
 {
     spin_lock(&dentry->d_lock);
     __d_drop(dentry);
     spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(d_drop);
 
 static inline void dentry_unlist(struct dentry *dentry, struct dentry *parent)
 {
     struct dentry *next;
     /*
      * Inform d_walk() and shrink_dentry_list() that we are no longer
      * attached to the dentry tree
      */
     dentry->d_flags |= DCACHE_DENTRY_KILLED;
     if (unlikely(list_empty(&dentry->d_child)))
         return;
     __list_del_entry(&dentry->d_child);
     /*
      * Cursors can move around the list of children.  While we'd been
      * a normal list member, it didn't matter - ->d_child.next would've
      * been updated.  However, from now on it won't be and for the
      * things like d_walk() it might end up with a nasty surprise.
      * Normally d_walk() doesn't care about cursors moving around -
      * ->d_lock on parent prevents that and since a cursor has no children
      * of its own, we get through it without ever unlocking the parent.
      * There is one exception, though - if we ascend from a child that
      * gets killed as soon as we unlock it, the next sibling is found
      * using the value left in its ->d_child.next.  And if _that_
      * pointed to a cursor, and cursor got moved (e.g. by lseek())
      * before d_walk() regains parent->d_lock, we'll end up skipping
      * everything the cursor had been moved past.
      *
      * Solution: make sure that the pointer left behind in ->d_child.next
      * points to something that won't be moving around.  I.e. skip the
      * cursors.
      */
     while (dentry->d_child.next != &parent->d_subdirs) {
         next = list_entry(dentry->d_child.next, struct dentry, d_child);
         if (likely(!(next->d_flags & DCACHE_DENTRY_CURSOR)))
             break;
         dentry->d_child.next = next->d_child.next;
     }
 }
 
 static void __dentry_kill(struct dentry *dentry)
 {
     struct dentry *parent = NULL;
     bool can_free = true;
     if (!IS_ROOT(dentry))
         parent = dentry->d_parent;
 
     /*
      * The dentry is now unrecoverably dead to the world.
      */
     lockref_mark_dead(&dentry->d_lockref);
 
     /*
      * inform the fs via d_prune that this dentry is about to be
      * unhashed and destroyed.
      */
     if (dentry->d_flags & DCACHE_OP_PRUNE)
         dentry->d_op->d_prune(dentry);
 
     if (dentry->d_flags & DCACHE_LRU_LIST) {
         if (!(dentry->d_flags & DCACHE_SHRINK_LIST))
             d_lru_del(dentry);
     }
     /* if it was on the hash then remove it */
     __d_drop(dentry);
     dentry_unlist(dentry, parent);
     if (parent)
         spin_unlock(&parent->d_lock);
     if (dentry->d_inode)
         dentry_unlink_inode(dentry);
     else
         spin_unlock(&dentry->d_lock);
     this_cpu_dec(nr_dentry);
     if (dentry->d_op && dentry->d_op->d_release)
         dentry->d_op->d_release(dentry);
 
     spin_lock(&dentry->d_lock);
     if (dentry->d_flags & DCACHE_SHRINK_LIST) {
         dentry->d_flags |= DCACHE_MAY_FREE;
         can_free = false;
     }
     spin_unlock(&dentry->d_lock);
     if (likely(can_free))
         dentry_free(dentry);
     cond_resched();
 }
 
 static struct dentry *__lock_parent(struct dentry *dentry)
 {
     struct dentry *parent;
     rcu_read_lock();
     spin_unlock(&dentry->d_lock);
 again:
     parent = READ_ONCE(dentry->d_parent);
     spin_lock(&parent->d_lock);
     /*
      * We can't blindly lock dentry until we are sure
      * that we won't violate the locking order.
      * Any changes of dentry->d_parent must have
      * been done with parent->d_lock held, so
      * spin_lock() above is enough of a barrier
      * for checking if it's still our child.
      */
     if (unlikely(parent != dentry->d_parent)) {
         spin_unlock(&parent->d_lock);
         goto again;
     }
     rcu_read_unlock();
     if (parent != dentry)
         spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
     else
         parent = NULL;
     return parent;
 }
 
 static inline struct dentry *lock_parent(struct dentry *dentry)
 {
     struct dentry *parent = dentry->d_parent;
     if (IS_ROOT(dentry))
         return NULL;
     if (likely(spin_trylock(&parent->d_lock)))
         return parent;
     return __lock_parent(dentry);
 }
 
 static inline bool retain_dentry(struct dentry *dentry)
 {
     WARN_ON(d_in_lookup(dentry));
 
     /* Unreachable? Get rid of it */
     if (unlikely(d_unhashed(dentry)))
         return false;
 
     if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
         return false;
 
     if (unlikely(dentry->d_flags & DCACHE_OP_DELETE)) {
         if (dentry->d_op->d_delete(dentry))
             return false;
     }
 
     if (unlikely(dentry->d_flags & DCACHE_DONTCACHE))
         return false;
 
     /* retain; LRU fodder */
     dentry->d_lockref.count--;
     if (unlikely(!(dentry->d_flags & DCACHE_LRU_LIST)))
         d_lru_add(dentry);
     else if (unlikely(!(dentry->d_flags & DCACHE_REFERENCED)))
         dentry->d_flags |= DCACHE_REFERENCED;
     return true;
 }
 
 void d_mark_dontcache(struct inode *inode)
 {
     struct dentry *de;
 
     spin_lock(&inode->i_lock);
     hlist_for_each_entry(de, &inode->i_dentry, d_u.d_alias) {
         spin_lock(&de->d_lock);
         de->d_flags |= DCACHE_DONTCACHE;
         spin_unlock(&de->d_lock);
     }
     inode->i_state |= I_DONTCACHE;
     spin_unlock(&inode->i_lock);
 }
 EXPORT_SYMBOL(d_mark_dontcache);
 
 /*
  * Finish off a dentry we've decided to kill.
  * dentry->d_lock must be held, returns with it unlocked.
  * Returns dentry requiring refcount drop, or NULL if we're done.
  */
 static struct dentry *dentry_kill(struct dentry *dentry)
     __releases(dentry->d_lock)
 {
     struct inode *inode = dentry->d_inode;
     struct dentry *parent = NULL;
 
     if (inode && unlikely(!spin_trylock(&inode->i_lock)))
         goto slow_positive;
 
     if (!IS_ROOT(dentry)) {
         parent = dentry->d_parent;
         if (unlikely(!spin_trylock(&parent->d_lock))) {
             parent = __lock_parent(dentry);
             if (likely(inode || !dentry->d_inode))
                 goto got_locks;
             /* negative that became positive */
             if (parent)
                 spin_unlock(&parent->d_lock);
             inode = dentry->d_inode;
             goto slow_positive;
         }
     }
     __dentry_kill(dentry);
     return parent;
 
 slow_positive:
     spin_unlock(&dentry->d_lock);
     spin_lock(&inode->i_lock);
     spin_lock(&dentry->d_lock);
     parent = lock_parent(dentry);
 got_locks:
     if (unlikely(dentry->d_lockref.count != 1)) {
         dentry->d_lockref.count--;
     } else if (likely(!retain_dentry(dentry))) {
         __dentry_kill(dentry);
         return parent;
     }
     /* we are keeping it, after all */
     if (inode)
         spin_unlock(&inode->i_lock);
     if (parent)
         spin_unlock(&parent->d_lock);
     spin_unlock(&dentry->d_lock);
     return NULL;
 }
 
 /*
  * Try to do a lockless dput(), and return whether that was successful.
  *
  * If unsuccessful, we return false, having already taken the dentry lock.
  *
  * The caller needs to hold the RCU read lock, so that the dentry is
  * guaranteed to stay around even if the refcount goes down to zero!
  */
 static inline bool fast_dput(struct dentry *dentry)
 {
     int ret;
     unsigned int d_flags;
 
     /*
      * If we have a d_op->d_delete() operation, we sould not
      * let the dentry count go to zero, so use "put_or_lock".
      */
     if (unlikely(dentry->d_flags & DCACHE_OP_DELETE))
         return lockref_put_or_lock(&dentry->d_lockref);
 
     /*
      * .. otherwise, we can try to just decrement the
      * lockref optimistically.
      */
     ret = lockref_put_return(&dentry->d_lockref);
 
     /*
      * If the lockref_put_return() failed due to the lock being held
      * by somebody else, the fast path has failed. We will need to
      * get the lock, and then check the count again.
      */
     if (unlikely(ret < 0)) {
         spin_lock(&dentry->d_lock);
         if (dentry->d_lockref.count > 1) {
             dentry->d_lockref.count--;
             spin_unlock(&dentry->d_lock);
             return true;
         }
         return false;
     }
 
     /*
      * If we weren't the last ref, we're done.
      */
     if (ret)
         return true;
 
     /*
      * Careful, careful. The reference count went down
      * to zero, but we don't hold the dentry lock, so
      * somebody else could get it again, and do another
      * dput(), and we need to not race with that.
      *
      * However, there is a very special and common case
      * where we don't care, because there is nothing to
      * do: the dentry is still hashed, it does not have
      * a 'delete' op, and it's referenced and already on
      * the LRU list.
      *
      * NOTE! Since we aren't locked, these values are
      * not "stable". However, it is sufficient that at
      * some point after we dropped the reference the
      * dentry was hashed and the flags had the proper
      * value. Other dentry users may have re-gotten
      * a reference to the dentry and change that, but
      * our work is done - we can leave the dentry
      * around with a zero refcount.
      *
      * Nevertheless, there are two cases that we should kill
      * the dentry anyway.
      * 1. free disconnected dentries as soon as their refcount
      *    reached zero.
      * 2. free dentries if they should not be cached.
      */
     smp_rmb();
     d_flags = READ_ONCE(dentry->d_flags);
     d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST |
             DCACHE_DISCONNECTED | DCACHE_DONTCACHE;
 
     /* Nothing to do? Dropping the reference was all we needed? */
     if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry))
         return true;
 
     /*
      * Not the fast normal case? Get the lock. We've already decremented
      * the refcount, but we'll need to re-check the situation after
      * getting the lock.
      */
     spin_lock(&dentry->d_lock);
 
     /*
      * Did somebody else grab a reference to it in the meantime, and
      * we're no longer the last user after all? Alternatively, somebody
      * else could have killed it and marked it dead. Either way, we
      * don't need to do anything else.
      */
     if (dentry->d_lockref.count) {
         spin_unlock(&dentry->d_lock);
         return true;
     }
 
     /*
      * Re-get the reference we optimistically dropped. We hold the
      * lock, and we just tested that it was zero, so we can just
      * set it to 1.
      */
     dentry->d_lockref.count = 1;
     return false;
 }
 
 
 /* 
  * This is dput
  *
  * This is complicated by the fact that we do not want to put
  * dentries that are no longer on any hash chain on the unused
  * list: we'd much rather just get rid of them immediately.
  *
  * However, that implies that we have to traverse the dentry
  * tree upwards to the parents which might _also_ now be
  * scheduled for deletion (it may have been only waiting for
  * its last child to go away).
  *
  * This tail recursion is done by hand as we don't want to depend
  * on the compiler to always get this right (gcc generally doesn't).
  * Real recursion would eat up our stack space.
  */
 
 /*
  * dput - release a dentry
  * @dentry: dentry to release 
  *
  * Release a dentry. This will drop the usage count and if appropriate
  * call the dentry unlink method as well as removing it from the queues and
  * releasing its resources. If the parent dentries were scheduled for release
  * they too may now get deleted.
  */
 void dput(struct dentry *dentry)
 {
     while (dentry) {
         might_sleep();
 
         rcu_read_lock();
         if (likely(fast_dput(dentry))) {
             rcu_read_unlock();
             return;
         }
 
         /* Slow case: now with the dentry lock held */
         rcu_read_unlock();
 
         if (likely(retain_dentry(dentry))) {
             spin_unlock(&dentry->d_lock);
             return;
         }
 
         dentry = dentry_kill(dentry);
     }
 }
 EXPORT_SYMBOL(dput);
 
 static void __dput_to_list(struct dentry *dentry, struct list_head *list)
 __must_hold(&dentry->d_lock)
 {
     if (dentry->d_flags & DCACHE_SHRINK_LIST) {
         /* let the owner of the list it's on deal with it */
         --dentry->d_lockref.count;
     } else {
         if (dentry->d_flags & DCACHE_LRU_LIST)
             d_lru_del(dentry);
         if (!--dentry->d_lockref.count)
             d_shrink_add(dentry, list);
     }
 }
 
 void dput_to_list(struct dentry *dentry, struct list_head *list)
 {
     rcu_read_lock();
     if (likely(fast_dput(dentry))) {
         rcu_read_unlock();
         return;
     }
     rcu_read_unlock();
     if (!retain_dentry(dentry))
         __dput_to_list(dentry, list);
     spin_unlock(&dentry->d_lock);
 }
 
 /* This must be called with d_lock held */
 static inline void __dget_dlock(struct dentry *dentry)
 {
     dentry->d_lockref.count++;
 }
 
 static inline void __dget(struct dentry *dentry)
 {
     lockref_get(&dentry->d_lockref);
 }
 
 struct dentry *dget_parent(struct dentry *dentry)
 {
     int gotref;
     struct dentry *ret;
     unsigned seq;
 
     /*
      * Do optimistic parent lookup without any
      * locking.
      */
     rcu_read_lock();
     seq = raw_seqcount_begin(&dentry->d_seq);
     ret = READ_ONCE(dentry->d_parent);
     gotref = lockref_get_not_zero(&ret->d_lockref);
     rcu_read_unlock();
     if (likely(gotref)) {
         if (!read_seqcount_retry(&dentry->d_seq, seq))
             return ret;
         dput(ret);
     }
 
 repeat:
     /*
      * Don't need rcu_dereference because we re-check it was correct under
      * the lock.
      */
     rcu_read_lock();
     ret = dentry->d_parent;
     spin_lock(&ret->d_lock);
     if (unlikely(ret != dentry->d_parent)) {
         spin_unlock(&ret->d_lock);
         rcu_read_unlock();
         goto repeat;
     }
     rcu_read_unlock();
     BUG_ON(!ret->d_lockref.count);
     ret->d_lockref.count++;
     spin_unlock(&ret->d_lock);
     return ret;
 }
 EXPORT_SYMBOL(dget_parent);
 
 static struct dentry * __d_find_any_alias(struct inode *inode)
 {
     struct dentry *alias;
 
     if (hlist_empty(&inode->i_dentry))
         return NULL;
     alias = hlist_entry(inode->i_dentry.first, struct dentry, d_u.d_alias);
     __dget(alias);
     return alias;
 }
 
 /**
  * d_find_any_alias - find any alias for a given inode
  * @inode: inode to find an alias for
  *
  * If any aliases exist for the given inode, take and return a
  * reference for one of them.  If no aliases exist, return %NULL.
  */
 struct dentry *d_find_any_alias(struct inode *inode)
 {
     struct dentry *de;
 
     spin_lock(&inode->i_lock);
     de = __d_find_any_alias(inode);
     spin_unlock(&inode->i_lock);
     return de;
 }
 EXPORT_SYMBOL(d_find_any_alias);
 
 static struct dentry *__d_find_alias(struct inode *inode)
 {
     struct dentry *alias;
 
     if (S_ISDIR(inode->i_mode))
         return __d_find_any_alias(inode);
 
     hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
         spin_lock(&alias->d_lock);
         if (!d_unhashed(alias)) {
             __dget_dlock(alias);
             spin_unlock(&alias->d_lock);
             return alias;
         }
         spin_unlock(&alias->d_lock);
     }
     return NULL;
 }
 
 /**
  * d_find_alias - grab a hashed alias of inode
  * @inode: inode in question
  *
  * If inode has a hashed alias, or is a directory and has any alias,
  * acquire the reference to alias and return it. Otherwise return NULL.
  * Notice that if inode is a directory there can be only one alias and
  * it can be unhashed only if it has no children, or if it is the root
  * of a filesystem, or if the directory was renamed and d_revalidate
  * was the first vfs operation to notice.
  *
  * If the inode has an IS_ROOT, DCACHE_DISCONNECTED alias, then prefer
  * any other hashed alias over that one.
  */
 struct dentry *d_find_alias(struct inode *inode)
 {
     struct dentry *de = NULL;
 
     if (!hlist_empty(&inode->i_dentry)) {
         spin_lock(&inode->i_lock);
         de = __d_find_alias(inode);
         spin_unlock(&inode->i_lock);
     }
     return de;
 }
 EXPORT_SYMBOL(d_find_alias);
 
 /*
  *  Caller MUST be holding rcu_read_lock() and be guaranteed
  *  that inode won't get freed until rcu_read_unlock().
  */
 struct dentry *d_find_alias_rcu(struct inode *inode)
 {
     struct hlist_head *l = &inode->i_dentry;
     struct dentry *de = NULL;
 
     spin_lock(&inode->i_lock);
     // ->i_dentry and ->i_rcu are colocated, but the latter won't be
     // used without having I_FREEING set, which means no aliases left
     if (likely(!(inode->i_state & I_FREEING) && !hlist_empty(l))) {
         if (S_ISDIR(inode->i_mode)) {
             de = hlist_entry(l->first, struct dentry, d_u.d_alias);
         } else {
             hlist_for_each_entry(de, l, d_u.d_alias)
                 if (!d_unhashed(de))
                     break;
         }
     }
     spin_unlock(&inode->i_lock);
     return de;
 }
 
 /*
  *  Try to kill dentries associated with this inode.
  * WARNING: you must own a reference to inode.
  */
 void d_prune_aliases(struct inode *inode)
 {
     struct dentry *dentry;
 restart:
     spin_lock(&inode->i_lock);
     hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias) {
         spin_lock(&dentry->d_lock);
         if (!dentry->d_lockref.count) {
             struct dentry *parent = lock_parent(dentry);
             if (likely(!dentry->d_lockref.count)) {
                 __dentry_kill(dentry);
                 dput(parent);
                 goto restart;
             }
             if (parent)
                 spin_unlock(&parent->d_lock);
         }
         spin_unlock(&dentry->d_lock);
     }
     spin_unlock(&inode->i_lock);
 }
 EXPORT_SYMBOL(d_prune_aliases);
 
 /*
  * Lock a dentry from shrink list.
  * Called under rcu_read_lock() and dentry->d_lock; the former
  * guarantees that nothing we access will be freed under us.
  * Note that dentry is *not* protected from concurrent dentry_kill(),
  * d_delete(), etc.
  *
  * Return false if dentry has been disrupted or grabbed, leaving
  * the caller to kick it off-list.  Otherwise, return true and have
  * that dentry's inode and parent both locked.
  */
 static bool shrink_lock_dentry(struct dentry *dentry)
 {
     struct inode *inode;
     struct dentry *parent;
 
     if (dentry->d_lockref.count)
         return false;
 
     inode = dentry->d_inode;
     if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
         spin_unlock(&dentry->d_lock);
         spin_lock(&inode->i_lock);
         spin_lock(&dentry->d_lock);
         if (unlikely(dentry->d_lockref.count))
             goto out;
         /* changed inode means that somebody had grabbed it */
         if (unlikely(inode != dentry->d_inode))
             goto out;
     }
 
     parent = dentry->d_parent;
     if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock)))
         return true;
 
     spin_unlock(&dentry->d_lock);
     spin_lock(&parent->d_lock);
     if (unlikely(parent != dentry->d_parent)) {
         spin_unlock(&parent->d_lock);
         spin_lock(&dentry->d_lock);
         goto out;
     }
     spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
     if (likely(!dentry->d_lockref.count))
         return true;
     spin_unlock(&parent->d_lock);
 out:
     if (inode)
         spin_unlock(&inode->i_lock);
     return false;
 }
 
 void shrink_dentry_list(struct list_head *list)
 {
     while (!list_empty(list)) {
         struct dentry *dentry, *parent;
 
         dentry = list_entry(list->prev, struct dentry, d_lru);
         spin_lock(&dentry->d_lock);
         rcu_read_lock();
         if (!shrink_lock_dentry(dentry)) {
             bool can_free = false;
             rcu_read_unlock();
             d_shrink_del(dentry);
             if (dentry->d_lockref.count < 0)
                 can_free = dentry->d_flags & DCACHE_MAY_FREE;
             spin_unlock(&dentry->d_lock);
             if (can_free)
                 dentry_free(dentry);
             continue;
         }
         rcu_read_unlock();
         d_shrink_del(dentry);
         parent = dentry->d_parent;
         if (parent != dentry)
             __dput_to_list(parent, list);
         __dentry_kill(dentry);
     }
 }
 
 static enum lru_status dentry_lru_isolate(struct list_head *item,
         struct list_lru_one *lru, spinlock_t *lru_lock, void *arg)
 {
     struct list_head *freeable = arg;
     struct dentry   *dentry = container_of(item, struct dentry, d_lru);
 
 
     /*
      * we are inverting the lru lock/dentry->d_lock here,
      * so use a trylock. If we fail to get the lock, just skip
      * it
      */
     if (!spin_trylock(&dentry->d_lock))
         return LRU_SKIP;
 
     /*
      * Referenced dentries are still in use. If they have active
      * counts, just remove them from the LRU. Otherwise give them
      * another pass through the LRU.
      */
     if (dentry->d_lockref.count) {
         d_lru_isolate(lru, dentry);
         spin_unlock(&dentry->d_lock);
         return LRU_REMOVED;
     }
 
     if (dentry->d_flags & DCACHE_REFERENCED) {
         dentry->d_flags &= ~DCACHE_REFERENCED;
         spin_unlock(&dentry->d_lock);
 
         /*
          * The list move itself will be made by the common LRU code. At
          * this point, we've dropped the dentry->d_lock but keep the
          * lru lock. This is safe to do, since every list movement is
          * protected by the lru lock even if both locks are held.
          *
          * This is guaranteed by the fact that all LRU management
          * functions are intermediated by the LRU API calls like
          * list_lru_add and list_lru_del. List movement in this file
          * only ever occur through this functions or through callbacks
          * like this one, that are called from the LRU API.
          *
          * The only exceptions to this are functions like
          * shrink_dentry_list, and code that first checks for the
          * DCACHE_SHRINK_LIST flag.  Those are guaranteed to be
          * operating only with stack provided lists after they are
          * properly isolated from the main list.  It is thus, always a
          * local access.
          */
         return LRU_ROTATE;
     }
 
     d_lru_shrink_move(lru, dentry, freeable);
     spin_unlock(&dentry->d_lock);
 
     return LRU_REMOVED;
 }
 
 /**
  * prune_dcache_sb - shrink the dcache
  * @sb: superblock
  * @sc: shrink control, passed to list_lru_shrink_walk()
  *
  * Attempt to shrink the superblock dcache LRU by @sc->nr_to_scan entries. This
  * is done when we need more memory and called from the superblock shrinker
  * function.
  *
  * This function may fail to free any resources if all the dentries are in
  * use.
  */
 long prune_dcache_sb(struct super_block *sb, struct shrink_control *sc)
 {
     LIST_HEAD(dispose);
     long freed;
 
     freed = list_lru_shrink_walk(&sb->s_dentry_lru, sc,
                      dentry_lru_isolate, &dispose);
     shrink_dentry_list(&dispose);
     return freed;
 }
 
 static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
         struct list_lru_one *lru, spinlock_t *lru_lock, void *arg)
 {
     struct list_head *freeable = arg;
     struct dentry   *dentry = container_of(item, struct dentry, d_lru);
 
     /*
      * we are inverting the lru lock/dentry->d_lock here,
      * so use a trylock. If we fail to get the lock, just skip
      * it
      */
     if (!spin_trylock(&dentry->d_lock))
         return LRU_SKIP;
 
     d_lru_shrink_move(lru, dentry, freeable);
     spin_unlock(&dentry->d_lock);
 
     return LRU_REMOVED;
 }
 
 
 /**
  * shrink_dcache_sb - shrink dcache for a superblock
  * @sb: superblock
  *
  * Shrink the dcache for the specified super block. This is used to free
  * the dcache before unmounting a file system.
  */
 void shrink_dcache_sb(struct super_block *sb)
 {
     do {
         LIST_HEAD(dispose);
 
         list_lru_walk(&sb->s_dentry_lru,
             dentry_lru_isolate_shrink, &dispose, 1024);
         shrink_dentry_list(&dispose);
     } while (list_lru_count(&sb->s_dentry_lru) > 0);
 }
 EXPORT_SYMBOL(shrink_dcache_sb);
 
 /**
  * enum d_walk_ret - action to talke during tree walk
  * @D_WALK_CONTINUE:    contrinue walk
  * @D_WALK_QUIT:    quit walk
  * @D_WALK_NORETRY: quit when retry is needed
  * @D_WALK_SKIP:    skip this dentry and its children
  */
 enum d_walk_ret {
     D_WALK_CONTINUE,
     D_WALK_QUIT,
     D_WALK_NORETRY,
     D_WALK_SKIP,
 };
 
 /**
  * d_walk - walk the dentry tree
  * @parent: start of walk
  * @data:   data passed to @enter() and @finish()
  * @enter:  callback when first entering the dentry
  *
  * The @enter() callbacks are called with d_lock held.
  */
 static void d_walk(struct dentry *parent, void *data,
            enum d_walk_ret (*enter)(void *, struct dentry *))
 {
     struct dentry *this_parent;
     struct list_head *next;
     unsigned seq = 0;
     enum d_walk_ret ret;
     bool retry = true;
 
 again:
     read_seqbegin_or_lock(&rename_lock, &seq);
     this_parent = parent;
     spin_lock(&this_parent->d_lock);
 
     ret = enter(data, this_parent);
     switch (ret) {
     case D_WALK_CONTINUE:
         break;
     case D_WALK_QUIT:
     case D_WALK_SKIP:
         goto out_unlock;
     case D_WALK_NORETRY:
         retry = false;
         break;
     }
 repeat:
     next = this_parent->d_subdirs.next;
 resume:
     while (next != &this_parent->d_subdirs) {
         struct list_head *tmp = next;
         struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
         next = tmp->next;
 
         if (unlikely(dentry->d_flags & DCACHE_DENTRY_CURSOR))
             continue;
 
         spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 
         ret = enter(data, dentry);
         switch (ret) {
         case D_WALK_CONTINUE:
             break;
         case D_WALK_QUIT:
             spin_unlock(&dentry->d_lock);
             goto out_unlock;
         case D_WALK_NORETRY:
             retry = false;
             break;
         case D_WALK_SKIP:
             spin_unlock(&dentry->d_lock);
             continue;
         }
 
         if (!list_empty(&dentry->d_subdirs)) {
             spin_unlock(&this_parent->d_lock);
             spin_release(&dentry->d_lock.dep_map, _RET_IP_);
             this_parent = dentry;
             spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
             goto repeat;
         }
         spin_unlock(&dentry->d_lock);
     }
     /*
      * All done at this level ... ascend and resume the search.
      */
     rcu_read_lock();
 ascend:
     if (this_parent != parent) {
         struct dentry *child = this_parent;
         this_parent = child->d_parent;
 
         spin_unlock(&child->d_lock);
         spin_lock(&this_parent->d_lock);
 
         /* might go back up the wrong parent if we have had a rename. */
         if (need_seqretry(&rename_lock, seq))
             goto rename_retry;
         /* go into the first sibling still alive */
         do {
             next = child->d_child.next;
             if (next == &this_parent->d_subdirs)
                 goto ascend;
             child = list_entry(next, struct dentry, d_child);
         } while (unlikely(child->d_flags & DCACHE_DENTRY_KILLED));
         rcu_read_unlock();
         goto resume;
     }
     if (need_seqretry(&rename_lock, seq))
         goto rename_retry;
     rcu_read_unlock();
 
 out_unlock:
     spin_unlock(&this_parent->d_lock);
     done_seqretry(&rename_lock, seq);
     return;
 
 rename_retry:
     spin_unlock(&this_parent->d_lock);
     rcu_read_unlock();
     BUG_ON(seq & 1);
     if (!retry)
         return;
     seq = 1;
     goto again;
 }
 
 struct check_mount {
     struct vfsmount *mnt;
     unsigned int mounted;
 };
 
 static enum d_walk_ret path_check_mount(void *data, struct dentry *dentry)
 {
     struct check_mount *info = data;
     struct path path = { .mnt = info->mnt, .dentry = dentry };
 
     if (likely(!d_mountpoint(dentry)))
         return D_WALK_CONTINUE;
     if (__path_is_mountpoint(&path)) {
         info->mounted = 1;
         return D_WALK_QUIT;
     }
     return D_WALK_CONTINUE;
 }
 
 /**
  * path_has_submounts - check for mounts over a dentry in the
  *                      current namespace.
  * @parent: path to check.
  *
  * Return true if the parent or its subdirectories contain
  * a mount point in the current namespace.
  */
 int path_has_submounts(const struct path *parent)
 {
     struct check_mount data = { .mnt = parent->mnt, .mounted = 0 };
 
     read_seqlock_excl(&mount_lock);
     d_walk(parent->dentry, &data, path_check_mount);
     read_sequnlock_excl(&mount_lock);
 
     return data.mounted;
 }
 EXPORT_SYMBOL(path_has_submounts);
 
 /*
  * Called by mount code to set a mountpoint and check if the mountpoint is
  * reachable (e.g. NFS can unhash a directory dentry and then the complete
  * subtree can become unreachable).
  *
  * Only one of d_invalidate() and d_set_mounted() must succeed.  For
  * this reason take rename_lock and d_lock on dentry and ancestors.
  */
 int d_set_mounted(struct dentry *dentry)
 {
     struct dentry *p;
     int ret = -ENOENT;
     write_seqlock(&rename_lock);
     for (p = dentry->d_parent; !IS_ROOT(p); p = p->d_parent) {
         /* Need exclusion wrt. d_invalidate() */
         spin_lock(&p->d_lock);
         if (unlikely(d_unhashed(p))) {
             spin_unlock(&p->d_lock);
             goto out;
         }
         spin_unlock(&p->d_lock);
     }
     spin_lock(&dentry->d_lock);
     if (!d_unlinked(dentry)) {
         ret = -EBUSY;
         if (!d_mountpoint(dentry)) {
             dentry->d_flags |= DCACHE_MOUNTED;
             ret = 0;
         }
     }
     spin_unlock(&dentry->d_lock);
 out:
     write_sequnlock(&rename_lock);
     return ret;
 }
 
 /*
  * Search the dentry child list of the specified parent,
  * and move any unused dentries to the end of the unused
  * list for prune_dcache(). We descend to the next level
  * whenever the d_subdirs list is non-empty and continue
  * searching.
  *
  * It returns zero iff there are no unused children,
  * otherwise  it returns the number of children moved to
  * the end of the unused list. This may not be the total
  * number of unused children, because select_parent can
  * drop the lock and return early due to latency
  * constraints.
  */
 
 struct select_data {
     struct dentry *start;
     union {
         long found;
         struct dentry *victim;
     };
     struct list_head dispose;
 };
 
 static enum d_walk_ret select_collect(void *_data, struct dentry *dentry)
 {
     struct select_data *data = _data;
     enum d_walk_ret ret = D_WALK_CONTINUE;
 
     if (data->start == dentry)
         goto out;
 
     if (dentry->d_flags & DCACHE_SHRINK_LIST) {
         data->found++;
     } else {
         if (dentry->d_flags & DCACHE_LRU_LIST)
             d_lru_del(dentry);
         if (!dentry->d_lockref.count) {
             d_shrink_add(dentry, &data->dispose);
             data->found++;
         }
     }
     /*
      * We can return to the caller if we have found some (this
      * ensures forward progress). We'll be coming back to find
      * the rest.
      */
     if (!list_empty(&data->dispose))
         ret = need_resched() ? D_WALK_QUIT : D_WALK_NORETRY;
 out:
     return ret;
 }
 
 static enum d_walk_ret select_collect2(void *_data, struct dentry *dentry)
 {
     struct select_data *data = _data;
     enum d_walk_ret ret = D_WALK_CONTINUE;
 
     if (data->start == dentry)
         goto out;
 
     if (dentry->d_flags & DCACHE_SHRINK_LIST) {
         if (!dentry->d_lockref.count) {
             rcu_read_lock();
             data->victim = dentry;
             return D_WALK_QUIT;
         }
     } else {
         if (dentry->d_flags & DCACHE_LRU_LIST)
             d_lru_del(dentry);
         if (!dentry->d_lockref.count)
             d_shrink_add(dentry, &data->dispose);
     }
     /*
      * We can return to the caller if we have found some (this
      * ensures forward progress). We'll be coming back to find
      * the rest.
      */
     if (!list_empty(&data->dispose))
         ret = need_resched() ? D_WALK_QUIT : D_WALK_NORETRY;
 out:
     return ret;
 }
 
 /**
  * shrink_dcache_parent - prune dcache
  * @parent: parent of entries to prune
  *
  * Prune the dcache to remove unused children of the parent dentry.
  */
 void shrink_dcache_parent(struct dentry *parent)
 {
     for (;;) {
         struct select_data data = {.start = parent};
 
         INIT_LIST_HEAD(&data.dispose);
         d_walk(parent, &data, select_collect);
 
         if (!list_empty(&data.dispose)) {
             shrink_dentry_list(&data.dispose);
             continue;
         }
 
         cond_resched();
         if (!data.found)
             break;
         data.victim = NULL;
         d_walk(parent, &data, select_collect2);
         if (data.victim) {
             struct dentry *parent;
             spin_lock(&data.victim->d_lock);
             if (!shrink_lock_dentry(data.victim)) {
                 spin_unlock(&data.victim->d_lock);
                 rcu_read_unlock();
             } else {
                 rcu_read_unlock();
                 parent = data.victim->d_parent;
                 if (parent != data.victim)
                     __dput_to_list(parent, &data.dispose);
                 __dentry_kill(data.victim);
             }
         }
         if (!list_empty(&data.dispose))
             shrink_dentry_list(&data.dispose);
     }
 }
 EXPORT_SYMBOL(shrink_dcache_parent);
 
 static enum d_walk_ret umount_check(void *_data, struct dentry *dentry)
 {
     /* it has busy descendents; complain about those instead */
     if (!list_empty(&dentry->d_subdirs))
         return D_WALK_CONTINUE;
 
     /* root with refcount 1 is fine */
     if (dentry == _data && dentry->d_lockref.count == 1)
         return D_WALK_CONTINUE;
 
     printk(KERN_ERR "BUG: Dentry %p{i=%lx,n=%pd} "
             " still in use (%d) [unmount of %s %s]\n",
                dentry,
                dentry->d_inode ?
                dentry->d_inode->i_ino : 0UL,
                dentry,
                dentry->d_lockref.count,
                dentry->d_sb->s_type->name,
                dentry->d_sb->s_id);
     WARN_ON(1);
     return D_WALK_CONTINUE;
 }
 
 static void do_one_tree(struct dentry *dentry)
 {
     shrink_dcache_parent(dentry);
     d_walk(dentry, dentry, umount_check);
     d_drop(dentry);
     dput(dentry);
 }
 
 /*
  * destroy the dentries attached to a superblock on unmounting
  */
 void shrink_dcache_for_umount(struct super_block *sb)
 {
     struct dentry *dentry;
 
     WARN(down_read_trylock(&sb->s_umount), "s_umount should've been locked");
 
     dentry = sb->s_root;
     sb->s_root = NULL;
     do_one_tree(dentry);
 
     while (!hlist_bl_empty(&sb->s_roots)) {
         dentry = dget(hlist_bl_entry(hlist_bl_first(&sb->s_roots), struct dentry, d_hash));
         do_one_tree(dentry);
     }
 }
 
 static enum d_walk_ret find_submount(void *_data, struct dentry *dentry)
 {
     struct dentry **victim = _data;
     if (d_mountpoint(dentry)) {
         __dget_dlock(dentry);
         *victim = dentry;
         return D_WALK_QUIT;
     }
     return D_WALK_CONTINUE;
 }
 
 /**
  * d_invalidate - detach submounts, prune dcache, and drop
  * @dentry: dentry to invalidate (aka detach, prune and drop)
  */
 void d_invalidate(struct dentry *dentry)
 {
     bool had_submounts = false;
     spin_lock(&dentry->d_lock);
     if (d_unhashed(dentry)) {
         spin_unlock(&dentry->d_lock);
         return;
     }
     __d_drop(dentry);
     spin_unlock(&dentry->d_lock);
 
     /* Negative dentries can be dropped without further checks */
     if (!dentry->d_inode)
         return;
 
     shrink_dcache_parent(dentry);
     for (;;) {
         struct dentry *victim = NULL;
         d_walk(dentry, &victim, find_submount);
         if (!victim) {
             if (had_submounts)
                 shrink_dcache_parent(dentry);
             return;
         }
         had_submounts = true;
         detach_mounts(victim);
         dput(victim);
     }
 }
 EXPORT_SYMBOL(d_invalidate);
 
 /**
  * __d_alloc    -   allocate a dcache entry
  * @sb: filesystem it will belong to
  * @name: qstr of the name
  *
  * Allocates a dentry. It returns %NULL if there is insufficient memory
  * available. On a success the dentry is returned. The name passed in is
  * copied and the copy passed in may be reused after this call.
  */
  
 static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 {
     struct dentry *dentry;
     char *dname;
     int err;
 
     dentry = kmem_cache_alloc_lru(dentry_cache, &sb->s_dentry_lru,
                       GFP_KERNEL);
     if (!dentry)
         return NULL;
 
     /*
      * We guarantee that the inline name is always NUL-terminated.
      * This way the memcpy() done by the name switching in rename
      * will still always have a NUL at the end, even if we might
      * be overwriting an internal NUL character
      */
     dentry->d_iname[DNAME_INLINE_LEN-1] = 0;
     if (unlikely(!name)) {
         name = &slash_name;
         dname = dentry->d_iname;
     } else if (name->len > DNAME_INLINE_LEN-1) {
         size_t size = offsetof(struct external_name, name[1]);
         struct external_name *p = kmalloc(size + name->len,
                           GFP_KERNEL_ACCOUNT |
                           __GFP_RECLAIMABLE);
         if (!p) {
             kmem_cache_free(dentry_cache, dentry); 
             return NULL;
         }
         atomic_set(&p->u.count, 1);
         dname = p->name;
     } else  {
         dname = dentry->d_iname;
     }   
 
     dentry->d_name.len = name->len;
     dentry->d_name.hash = name->hash;
     memcpy(dname, name->name, name->len);
     dname[name->len] = 0;
 
     /* Make sure we always see the terminating NUL character */
     smp_store_release(&dentry->d_name.name, dname); /* ^^^ */
 
     dentry->d_lockref.count = 1;
     dentry->d_flags = 0;
     spin_lock_init(&dentry->d_lock);
     seqcount_spinlock_init(&dentry->d_seq, &dentry->d_lock);
     dentry->d_inode = NULL;
     dentry->d_parent = dentry;
     dentry->d_sb = sb;
     dentry->d_op = NULL;
     dentry->d_fsdata = NULL;
     INIT_HLIST_BL_NODE(&dentry->d_hash);
     INIT_LIST_HEAD(&dentry->d_lru);
     INIT_LIST_HEAD(&dentry->d_subdirs);
     INIT_HLIST_NODE(&dentry->d_u.d_alias);
     INIT_LIST_HEAD(&dentry->d_child);
     d_set_d_op(dentry, dentry->d_sb->s_d_op);
 
     if (dentry->d_op && dentry->d_op->d_init) {
         err = dentry->d_op->d_init(dentry);
         if (err) {
             if (dname_external(dentry))
                 kfree(external_name(dentry));
             kmem_cache_free(dentry_cache, dentry);
             return NULL;
         }
     }
 
     this_cpu_inc(nr_dentry);
 
     return dentry;
 }
 
 /**
  * d_alloc  -   allocate a dcache entry
  * @parent: parent of entry to allocate
  * @name: qstr of the name
  *
  * Allocates a dentry. It returns %NULL if there is insufficient memory
  * available. On a success the dentry is returned. The name passed in is
  * copied and the copy passed in may be reused after this call.
  */
 struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
 {
     struct dentry *dentry = __d_alloc(parent->d_sb, name);
     if (!dentry)
         return NULL;
     spin_lock(&parent->d_lock);
     /*
      * don't need child lock because it is not subject
      * to concurrency here
      */
     __dget_dlock(parent);
     dentry->d_parent = parent;
     list_add(&dentry->d_child, &parent->d_subdirs);
     spin_unlock(&parent->d_lock);
 
     return dentry;
 }
 EXPORT_SYMBOL(d_alloc);
 
 struct dentry *d_alloc_anon(struct super_block *sb)
 {
     return __d_alloc(sb, NULL);
 }
 EXPORT_SYMBOL(d_alloc_anon);
 
 struct dentry *d_alloc_cursor(struct dentry * parent)
 {
     struct dentry *dentry = d_alloc_anon(parent->d_sb);
     if (dentry) {
         dentry->d_flags |= DCACHE_DENTRY_CURSOR;
         dentry->d_parent = dget(parent);
     }
     return dentry;
 }
 
 /**
  * d_alloc_pseudo - allocate a dentry (for lookup-less filesystems)
  * @sb: the superblock
  * @name: qstr of the name
  *
  * For a filesystem that just pins its dentries in memory and never
  * performs lookups at all, return an unhashed IS_ROOT dentry.
  * This is used for pipes, sockets et.al. - the stuff that should
  * never be anyone's children or parents.  Unlike all other
  * dentries, these will not have RCU delay between dropping the
  * last reference and freeing them.
  *
  * The only user is alloc_file_pseudo() and that's what should
  * be considered a public interface.  Don't use directly.
  */
 struct dentry *d_alloc_pseudo(struct super_block *sb, const struct qstr *name)
 {
     struct dentry *dentry = __d_alloc(sb, name);
     if (likely(dentry))
         dentry->d_flags |= DCACHE_NORCU;
     return dentry;
 }
 
 struct dentry *d_alloc_name(struct dentry *parent, const char *name)
 {
     struct qstr q;
 
     q.name = name;
     q.hash_len = hashlen_string(parent, name);
     return d_alloc(parent, &q);
 }
 EXPORT_SYMBOL(d_alloc_name);
 
 void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
 {
     WARN_ON_ONCE(dentry->d_op);
     WARN_ON_ONCE(dentry->d_flags & (DCACHE_OP_HASH  |
                 DCACHE_OP_COMPARE   |
                 DCACHE_OP_REVALIDATE    |
                 DCACHE_OP_WEAK_REVALIDATE   |
                 DCACHE_OP_DELETE    |
                 DCACHE_OP_REAL));
     dentry->d_op = op;
     if (!op)
         return;
     if (op->d_hash)
         dentry->d_flags |= DCACHE_OP_HASH;
     if (op->d_compare)
         dentry->d_flags |= DCACHE_OP_COMPARE;
     if (op->d_revalidate)
         dentry->d_flags |= DCACHE_OP_REVALIDATE;
     if (op->d_weak_revalidate)
         dentry->d_flags |= DCACHE_OP_WEAK_REVALIDATE;
     if (op->d_delete)
         dentry->d_flags |= DCACHE_OP_DELETE;
     if (op->d_prune)
         dentry->d_flags |= DCACHE_OP_PRUNE;
     if (op->d_real)
         dentry->d_flags |= DCACHE_OP_REAL;
 
 }
 EXPORT_SYMBOL(d_set_d_op);
 
 
 /*
  * d_set_fallthru - Mark a dentry as falling through to a lower layer
  * @dentry - The dentry to mark
  *
  * Mark a dentry as falling through to the lower layer (as set with
  * d_pin_lower()).  This flag may be recorded on the medium.
  */
 void d_set_fallthru(struct dentry *dentry)
 {
     spin_lock(&dentry->d_lock);
     dentry->d_flags |= DCACHE_FALLTHRU;
     spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(d_set_fallthru);
 
 static unsigned d_flags_for_inode(struct inode *inode)
 {
     unsigned add_flags = DCACHE_REGULAR_TYPE;
 
     if (!inode)
         return DCACHE_MISS_TYPE;
 
     if (S_ISDIR(inode->i_mode)) {
         add_flags = DCACHE_DIRECTORY_TYPE;
         if (unlikely(!(inode->i_opflags & IOP_LOOKUP))) {
             if (unlikely(!inode->i_op->lookup))
                 add_flags = DCACHE_AUTODIR_TYPE;
             else
                 inode->i_opflags |= IOP_LOOKUP;
         }
         goto type_determined;
     }
 
     if (unlikely(!(inode->i_opflags & IOP_NOFOLLOW))) {
         if (unlikely(inode->i_op->get_link)) {
             add_flags = DCACHE_SYMLINK_TYPE;
             goto type_determined;
         }
         inode->i_opflags |= IOP_NOFOLLOW;
     }
 
     if (unlikely(!S_ISREG(inode->i_mode)))
         add_flags = DCACHE_SPECIAL_TYPE;
 
 type_determined:
     if (unlikely(IS_AUTOMOUNT(inode)))
         add_flags |= DCACHE_NEED_AUTOMOUNT;
     return add_flags;
 }
 
 static void __d_instantiate(struct dentry *dentry, struct inode *inode)
 {
     unsigned add_flags = d_flags_for_inode(inode);
     WARN_ON(d_in_lookup(dentry));
 
     spin_lock(&dentry->d_lock);
     /*
      * Decrement negative dentry count if it was in the LRU list.
      */
     if (dentry->d_flags & DCACHE_LRU_LIST)
         this_cpu_dec(nr_dentry_negative);
     hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
     raw_write_seqcount_begin(&dentry->d_seq);
     __d_set_inode_and_type(dentry, inode, add_flags);
     raw_write_seqcount_end(&dentry->d_seq);
     fsnotify_update_flags(dentry);
     spin_unlock(&dentry->d_lock);
 }
 
 /**
  * d_instantiate - fill in inode information for a dentry
  * @entry: dentry to complete
  * @inode: inode to attach to this dentry
  *
  * Fill in inode information in the entry.
  *
  * This turns negative dentries into productive full members
  * of society.
  *
  * NOTE! This assumes that the inode count has been incremented
  * (or otherwise set) by the caller to indicate that it is now
  * in use by the dcache.
  */
  
 void d_instantiate(struct dentry *entry, struct inode * inode)
 {
     BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
     if (inode) {
         security_d_instantiate(entry, inode);
         spin_lock(&inode->i_lock);
         __d_instantiate(entry, inode);
         spin_unlock(&inode->i_lock);
     }
 }
 EXPORT_SYMBOL(d_instantiate);
 
 /*
  * This should be equivalent to d_instantiate() + unlock_new_inode(),
  * with lockdep-related part of unlock_new_inode() done before
  * anything else.  Use that instead of open-coding d_instantiate()/
  * unlock_new_inode() combinations.
  */
 void d_instantiate_new(struct dentry *entry, struct inode *inode)
 {
     BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
     BUG_ON(!inode);
     lockdep_annotate_inode_mutex_key(inode);
     security_d_instantiate(entry, inode);
     spin_lock(&inode->i_lock);
     __d_instantiate(entry, inode);
     WARN_ON(!(inode->i_state & I_NEW));
     inode->i_state &= ~I_NEW & ~I_CREATING;
     smp_mb();
     wake_up_bit(&inode->i_state, __I_NEW);
     spin_unlock(&inode->i_lock);
 }
 EXPORT_SYMBOL(d_instantiate_new);
 
 struct dentry *d_make_root(struct inode *root_inode)
 {
     struct dentry *res = NULL;
 
     if (root_inode) {
         res = d_alloc_anon(root_inode->i_sb);
         if (res)
             d_instantiate(res, root_inode);
         else
             iput(root_inode);
     }
     return res;
 }
 EXPORT_SYMBOL(d_make_root);
 
 static struct dentry *__d_instantiate_anon(struct dentry *dentry,
                        struct inode *inode,
                        bool disconnected)
 {
     struct dentry *res;
     unsigned add_flags;
 
     security_d_instantiate(dentry, inode);
     spin_lock(&inode->i_lock);
     res = __d_find_any_alias(inode);
     if (res) {
         spin_unlock(&inode->i_lock);
         dput(dentry);
         goto out_iput;
     }
 
     /* attach a disconnected dentry */
     add_flags = d_flags_for_inode(inode);
 
     if (disconnected)
         add_flags |= DCACHE_DISCONNECTED;
 
     spin_lock(&dentry->d_lock);
     __d_set_inode_and_type(dentry, inode, add_flags);
     hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
     if (!disconnected) {
         hlist_bl_lock(&dentry->d_sb->s_roots);
         hlist_bl_add_head(&dentry->d_hash, &dentry->d_sb->s_roots);
         hlist_bl_unlock(&dentry->d_sb->s_roots);
     }
     spin_unlock(&dentry->d_lock);
     spin_unlock(&inode->i_lock);
 
     return dentry;
 
  out_iput:
     iput(inode);
     return res;
 }
 
 struct dentry *d_instantiate_anon(struct dentry *dentry, struct inode *inode)
 {
     return __d_instantiate_anon(dentry, inode, true);
 }
 EXPORT_SYMBOL(d_instantiate_anon);
 
 static struct dentry *__d_obtain_alias(struct inode *inode, bool disconnected)
 {
     struct dentry *tmp;
     struct dentry *res;
 
     if (!inode)
         return ERR_PTR(-ESTALE);
     if (IS_ERR(inode))
         return ERR_CAST(inode);
 
     res = d_find_any_alias(inode);
     if (res)
         goto out_iput;
 
     tmp = d_alloc_anon(inode->i_sb);
     if (!tmp) {
         res = ERR_PTR(-ENOMEM);
         goto out_iput;
     }
 
     return __d_instantiate_anon(tmp, inode, disconnected);
 
 out_iput:
     iput(inode);
     return res;
 }
 
 /**
  * d_obtain_alias - find or allocate a DISCONNECTED dentry for a given inode
  * @inode: inode to allocate the dentry for
  *
  * Obtain a dentry for an inode resulting from NFS filehandle conversion or
  * similar open by handle operations.  The returned dentry may be anonymous,
  * or may have a full name (if the inode was already in the cache).
  *
  * When called on a directory inode, we must ensure that the inode only ever
  * has one dentry.  If a dentry is found, that is returned instead of
  * allocating a new one.
  *
  * On successful return, the reference to the inode has been transferred
  * to the dentry.  In case of an error the reference on the inode is released.
  * To make it easier to use in export operations a %NULL or IS_ERR inode may
  * be passed in and the error will be propagated to the return value,
  * with a %NULL @inode replaced by ERR_PTR(-ESTALE).
  */
 struct dentry *d_obtain_alias(struct inode *inode)
 {
     return __d_obtain_alias(inode, true);
 }
 EXPORT_SYMBOL(d_obtain_alias);
 
 /**
  * d_obtain_root - find or allocate a dentry for a given inode
  * @inode: inode to allocate the dentry for
  *
  * Obtain an IS_ROOT dentry for the root of a filesystem.
  *
  * We must ensure that directory inodes only ever have one dentry.  If a
  * dentry is found, that is returned instead of allocating a new one.
  *
  * On successful return, the reference to the inode has been transferred
  * to the dentry.  In case of an error the reference on the inode is
  * released.  A %NULL or IS_ERR inode may be passed in and will be the
  * error will be propagate to the return value, with a %NULL @inode
  * replaced by ERR_PTR(-ESTALE).
  */
 struct dentry *d_obtain_root(struct inode *inode)
 {
     return __d_obtain_alias(inode, false);
 }
 EXPORT_SYMBOL(d_obtain_root);
 
 /**
  * d_add_ci - lookup or allocate new dentry with case-exact name
  * @inode:  the inode case-insensitive lookup has found
  * @dentry: the negative dentry that was passed to the parent's lookup func
  * @name:   the case-exact name to be associated with the returned dentry
  *
  * This is to avoid filling the dcache with case-insensitive names to the
  * same inode, only the actual correct case is stored in the dcache for
  * case-insensitive filesystems.
  *
  * For a case-insensitive lookup match and if the case-exact dentry
  * already exists in the dcache, use it and return it.
  *
  * If no entry exists with the exact case name, allocate new dentry with
  * the exact case, and return the spliced entry.
  */
 struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
             struct qstr *name)
 {
     struct dentry *found, *res;
 
     /*
      * First check if a dentry matching the name already exists,
      * if not go ahead and create it now.
      */
     found = d_hash_and_lookup(dentry->d_parent, name);
     if (found) {
         iput(inode);
         return found;
     }
     if (d_in_lookup(dentry)) {
         found = d_alloc_parallel(dentry->d_parent, name,
                     dentry->d_wait);
         if (IS_ERR(found) || !d_in_lookup(found)) {
             iput(inode);
             return found;
         }
     } else {
         found = d_alloc(dentry->d_parent, name);
         if (!found) {
             iput(inode);
             return ERR_PTR(-ENOMEM);
         } 
     }
     res = d_splice_alias(inode, found);
     if (res) {
         d_lookup_done(found);
         dput(found);
         return res;
     }
     return found;
 }
 EXPORT_SYMBOL(d_add_ci);
 
 /**
  * d_same_name - compare dentry name with case-exact name
  * @parent: parent dentry
  * @dentry: the negative dentry that was passed to the parent's lookup func
  * @name:   the case-exact name to be associated with the returned dentry
  *
  * Return: true if names are same, or false
  */
 bool d_same_name(const struct dentry *dentry, const struct dentry *parent,
          const struct qstr *name)
 {
     if (likely(!(parent->d_flags & DCACHE_OP_COMPARE))) {
         if (dentry->d_name.len != name->len)
             return false;
         return dentry_cmp(dentry, name->name, name->len) == 0;
     }
     return parent->d_op->d_compare(dentry,
                        dentry->d_name.len, dentry->d_name.name,
                        name) == 0;
 }
 EXPORT_SYMBOL_GPL(d_same_name);
 
 /*
  * This is __d_lookup_rcu() when the parent dentry has
  * DCACHE_OP_COMPARE, which makes things much nastier.
  */
 static noinline struct dentry *__d_lookup_rcu_op_compare(
     const struct dentry *parent,
     const struct qstr *name,
     unsigned *seqp)
 {
     u64 hashlen = name->hash_len;
     struct hlist_bl_head *b = d_hash(hashlen_hash(hashlen));
     struct hlist_bl_node *node;
     struct dentry *dentry;
 
     hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
         int tlen;
         const char *tname;
         unsigned seq;
 
 seqretry:
         seq = raw_seqcount_begin(&dentry->d_seq);
         if (dentry->d_parent != parent)
             continue;
         if (d_unhashed(dentry))
             continue;
         if (dentry->d_name.hash != hashlen_hash(hashlen))
             continue;
         tlen = dentry->d_name.len;
         tname = dentry->d_name.name;
         /* we want a consistent (name,len) pair */
         if (read_seqcount_retry(&dentry->d_seq, seq)) {
             cpu_relax();
             goto seqretry;
         }
         if (parent->d_op->d_compare(dentry, tlen, tname, name) != 0)
             continue;
         *seqp = seq;
         return dentry;
     }
     return NULL;
 }
 
 /**
  * __d_lookup_rcu - search for a dentry (racy, store-free)
  * @parent: parent dentry
  * @name: qstr of name we wish to find
  * @seqp: returns d_seq value at the point where the dentry was found
  * Returns: dentry, or NULL
  *
  * __d_lookup_rcu is the dcache lookup function for rcu-walk name
  * resolution (store-free path walking) design described in
  * Documentation/filesystems/path-lookup.txt.
  *
  * This is not to be used outside core vfs.
  *
  * __d_lookup_rcu must only be used in rcu-walk mode, ie. with vfsmount lock
  * held, and rcu_read_lock held. The returned dentry must not be stored into
  * without taking d_lock and checking d_seq sequence count against @seq
  * returned here.
  *
  * A refcount may be taken on the found dentry with the d_rcu_to_refcount
  * function.
  *
  * Alternatively, __d_lookup_rcu may be called again to look up the child of
  * the returned dentry, so long as its parent's seqlock is checked after the
  * child is looked up. Thus, an interlocking stepping of sequence lock checks
  * is formed, giving integrity down the path walk.
  *
  * NOTE! The caller *has* to check the resulting dentry against the sequence
  * number we've returned before using any of the resulting dentry state!
  */
 struct dentry *__d_lookup_rcu(const struct dentry *parent,
                 const struct qstr *name,
                 unsigned *seqp)
 {
     u64 hashlen = name->hash_len;
     const unsigned char *str = name->name;
     struct hlist_bl_head *b = d_hash(hashlen_hash(hashlen));
     struct hlist_bl_node *node;
     struct dentry *dentry;
 
     /*
      * Note: There is significant duplication with __d_lookup_rcu which is
      * required to prevent single threaded performance regressions
      * especially on architectures where smp_rmb (in seqcounts) are costly.
      * Keep the two functions in sync.
      */
 
     if (unlikely(parent->d_flags & DCACHE_OP_COMPARE))
         return __d_lookup_rcu_op_compare(parent, name, seqp);
 
     /*
      * The hash list is protected using RCU.
      *
      * Carefully use d_seq when comparing a candidate dentry, to avoid
      * races with d_move().
      *
      * It is possible that concurrent renames can mess up our list
      * walk here and result in missing our dentry, resulting in the
      * false-negative result. d_lookup() protects against concurrent
      * renames using rename_lock seqlock.
      *
      * See Documentation/filesystems/path-lookup.txt for more details.
      */
     hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
         unsigned seq;
 
         /*
          * The dentry sequence count protects us from concurrent
          * renames, and thus protects parent and name fields.
          *
          * The caller must perform a seqcount check in order
          * to do anything useful with the returned dentry.
          *
          * NOTE! We do a "raw" seqcount_begin here. That means that
          * we don't wait for the sequence count to stabilize if it
          * is in the middle of a sequence change. If we do the slow
          * dentry compare, we will do seqretries until it is stable,
          * and if we end up with a successful lookup, we actually
          * want to exit RCU lookup anyway.
          *
          * Note that raw_seqcount_begin still *does* smp_rmb(), so
          * we are still guaranteed NUL-termination of ->d_name.name.
          */
         seq = raw_seqcount_begin(&dentry->d_seq);
         if (dentry->d_parent != parent)
             continue;
         if (d_unhashed(dentry))
             continue;
         if (dentry->d_name.hash_len != hashlen)
             continue;
         if (dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0)
             continue;
         *seqp = seq;
         return dentry;
     }
     return NULL;
 }
 
 /**
  * d_lookup - search for a dentry
  * @parent: parent dentry
  * @name: qstr of name we wish to find
  * Returns: dentry, or NULL
  *
  * d_lookup searches the children of the parent dentry for the name in
  * question. If the dentry is found its reference count is incremented and the
  * dentry is returned. The caller must use dput to free the entry when it has
  * finished using it. %NULL is returned if the dentry does not exist.
  */
 struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
 {
     struct dentry *dentry;
     unsigned seq;
 
     do {
         seq = read_seqbegin(&rename_lock);
         dentry = __d_lookup(parent, name);
         if (dentry)
             break;
     } while (read_seqretry(&rename_lock, seq));
     return dentry;
 }
 EXPORT_SYMBOL(d_lookup);
 
 /**
  * __d_lookup - search for a dentry (racy)
  * @parent: parent dentry
  * @name: qstr of name we wish to find
  * Returns: dentry, or NULL
  *
  * __d_lookup is like d_lookup, however it may (rarely) return a
  * false-negative result due to unrelated rename activity.
  *
  * __d_lookup is slightly faster by avoiding rename_lock read seqlock,
  * however it must be used carefully, eg. with a following d_lookup in
  * the case of failure.
  *
  * __d_lookup callers must be commented.
  */
 struct dentry *__d_lookup(const struct dentry *parent, const struct qstr *name)
 {
     unsigned int hash = name->hash;
     struct hlist_bl_head *b = d_hash(hash);
     struct hlist_bl_node *node;
     struct dentry *found = NULL;
     struct dentry *dentry;
 
     /*
      * Note: There is significant duplication with __d_lookup_rcu which is
      * required to prevent single threaded performance regressions
      * especially on architectures where smp_rmb (in seqcounts) are costly.
      * Keep the two functions in sync.
      */
 
     /*
      * The hash list is protected using RCU.
      *
      * Take d_lock when comparing a candidate dentry, to avoid races
      * with d_move().
      *
      * It is possible that concurrent renames can mess up our list
      * walk here and result in missing our dentry, resulting in the
      * false-negative result. d_lookup() protects against concurrent
      * renames using rename_lock seqlock.
      *
      * See Documentation/filesystems/path-lookup.txt for more details.
      */
     rcu_read_lock();
     
     hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
 
         if (dentry->d_name.hash != hash)
             continue;
 
         spin_lock(&dentry->d_lock);
         if (dentry->d_parent != parent)
             goto next;
         if (d_unhashed(dentry))
             goto next;
 
         if (!d_same_name(dentry, parent, name))
             goto next;
 
         dentry->d_lockref.count++;
         found = dentry;
         spin_unlock(&dentry->d_lock);
         break;
 next:
         spin_unlock(&dentry->d_lock);
     }
     rcu_read_unlock();
 
     return found;
 }
 
 /**
  * d_hash_and_lookup - hash the qstr then search for a dentry
  * @dir: Directory to search in
  * @name: qstr of name we wish to find
  *
  * On lookup failure NULL is returned; on bad name - ERR_PTR(-error)
  */
 struct dentry *d_hash_and_lookup(struct dentry *dir, struct qstr *name)
 {
     /*
      * Check for a fs-specific hash function. Note that we must
      * calculate the standard hash first, as the d_op->d_hash()
      * routine may choose to leave the hash value unchanged.
      */
     name->hash = full_name_hash(dir, name->name, name->len);
     if (dir->d_flags & DCACHE_OP_HASH) {
         int err = dir->d_op->d_hash(dir, name);
         if (unlikely(err < 0))
             return ERR_PTR(err);
     }
     return d_lookup(dir, name);
 }
 EXPORT_SYMBOL(d_hash_and_lookup);
 
 /*
  * When a file is deleted, we have two options:
  * - turn this dentry into a negative dentry
  * - unhash this dentry and free it.
  *
  * Usually, we want to just turn this into
  * a negative dentry, but if anybody else is
  * currently using the dentry or the inode
  * we can't do that and we fall back on removing
  * it from the hash queues and waiting for
  * it to be deleted later when it has no users
  */
  
 /**
  * d_delete - delete a dentry
  * @dentry: The dentry to delete
  *
  * Turn the dentry into a negative dentry if possible, otherwise
  * remove it from the hash queues so it can be deleted later
  */
  
 void d_delete(struct dentry * dentry)
 {
     struct inode *inode = dentry->d_inode;
 
     spin_lock(&inode->i_lock);
     spin_lock(&dentry->d_lock);
     /*
      * Are we the only user?
      */
     if (dentry->d_lockref.count == 1) {
         dentry->d_flags &= ~DCACHE_CANT_MOUNT;
         dentry_unlink_inode(dentry);
     } else {
         __d_drop(dentry);
         spin_unlock(&dentry->d_lock);
         spin_unlock(&inode->i_lock);
     }
 }
 EXPORT_SYMBOL(d_delete);
 
 static void __d_rehash(struct dentry *entry)
 {
     struct hlist_bl_head *b = d_hash(entry->d_name.hash);
 
     hlist_bl_lock(b);
     hlist_bl_add_head_rcu(&entry->d_hash, b);
     hlist_bl_unlock(b);
 }
 
 /**
  * d_rehash - add an entry back to the hash
  * @entry: dentry to add to the hash
  *
  * Adds a dentry to the hash according to its name.
  */
  
 void d_rehash(struct dentry * entry)
 {
     spin_lock(&entry->d_lock);
     __d_rehash(entry);
     spin_unlock(&entry->d_lock);
 }
 EXPORT_SYMBOL(d_rehash);
 
 static inline unsigned start_dir_add(struct inode *dir)
 {
     /*
      * The caller holds a spinlock (dentry::d_lock). On !PREEMPT_RT
      * kernels spin_lock() implicitly disables preemption, but not on
      * PREEMPT_RT.  So for RT it has to be done explicitly to protect
      * the sequence count write side critical section against a reader
      * or another writer preempting, which would result in a live lock.
      */
     if (IS_ENABLED(CONFIG_PREEMPT_RT))
         preempt_disable();
     for (;;) {
         unsigned n = dir->i_dir_seq;
         if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
             return n;
         cpu_relax();
     }
 }
 
 static inline void end_dir_add(struct inode *dir, unsigned int n,
                    wait_queue_head_t *d_wait)
 {
     smp_store_release(&dir->i_dir_seq, n + 2);
     if (IS_ENABLED(CONFIG_PREEMPT_RT))
         preempt_enable();
     wake_up_all(d_wait);
 }
 
 static void d_wait_lookup(struct dentry *dentry)
 {
     if (d_in_lookup(dentry)) {
         DECLARE_WAITQUEUE(wait, current);
         add_wait_queue(dentry->d_wait, &wait);
         do {
             set_current_state(TASK_UNINTERRUPTIBLE);
             spin_unlock(&dentry->d_lock);
             schedule();
             spin_lock(&dentry->d_lock);
         } while (d_in_lookup(dentry));
     }
 }
 
 struct dentry *d_alloc_parallel(struct dentry *parent,
                 const struct qstr *name,
                 wait_queue_head_t *wq)
 {
     unsigned int hash = name->hash;
     struct hlist_bl_head *b = in_lookup_hash(parent, hash);
     struct hlist_bl_node *node;
     struct dentry *new = d_alloc(parent, name);
     struct dentry *dentry;
     unsigned seq, r_seq, d_seq;
 
     if (unlikely(!new))
         return ERR_PTR(-ENOMEM);
 
 retry:
     rcu_read_lock();
     seq = smp_load_acquire(&parent->d_inode->i_dir_seq);
     r_seq = read_seqbegin(&rename_lock);
     dentry = __d_lookup_rcu(parent, name, &d_seq);
     if (unlikely(dentry)) {
         if (!lockref_get_not_dead(&dentry->d_lockref)) {
             rcu_read_unlock();
             goto retry;
         }
         if (read_seqcount_retry(&dentry->d_seq, d_seq)) {
             rcu_read_unlock();
             dput(dentry);
             goto retry;
         }
         rcu_read_unlock();
         dput(new);
         return dentry;
     }
     if (unlikely(read_seqretry(&rename_lock, r_seq))) {
         rcu_read_unlock();
         goto retry;
     }
 
     if (unlikely(seq & 1)) {
         rcu_read_unlock();
         goto retry;
     }
 
     hlist_bl_lock(b);
     if (unlikely(READ_ONCE(parent->d_inode->i_dir_seq) != seq)) {
         hlist_bl_unlock(b);
         rcu_read_unlock();
         goto retry;
     }
     /*
      * No changes for the parent since the beginning of d_lookup().
      * Since all removals from the chain happen with hlist_bl_lock(),
      * any potential in-lookup matches are going to stay here until
      * we unlock the chain.  All fields are stable in everything
      * we encounter.
      */
     hlist_bl_for_each_entry(dentry, node, b, d_u.d_in_lookup_hash) {
         if (dentry->d_name.hash != hash)
             continue;
         if (dentry->d_parent != parent)
             continue;
         if (!d_same_name(dentry, parent, name))
             continue;
         hlist_bl_unlock(b);
         /* now we can try to grab a reference */
         if (!lockref_get_not_dead(&dentry->d_lockref)) {
             rcu_read_unlock();
             goto retry;
         }
 
         rcu_read_unlock();
         /*
          * somebody is likely to be still doing lookup for it;
          * wait for them to finish
          */
         spin_lock(&dentry->d_lock);
         d_wait_lookup(dentry);
         /*
          * it's not in-lookup anymore; in principle we should repeat
          * everything from dcache lookup, but it's likely to be what
          * d_lookup() would've found anyway.  If it is, just return it;
          * otherwise we really have to repeat the whole thing.
          */
         if (unlikely(dentry->d_name.hash != hash))
             goto mismatch;
         if (unlikely(dentry->d_parent != parent))
             goto mismatch;
         if (unlikely(d_unhashed(dentry)))
             goto mismatch;
         if (unlikely(!d_same_name(dentry, parent, name)))
             goto mismatch;
         /* OK, it *is* a hashed match; return it */
         spin_unlock(&dentry->d_lock);
         dput(new);
         return dentry;
     }
     rcu_read_unlock();
     /* we can't take ->d_lock here; it's OK, though. */
     new->d_flags |= DCACHE_PAR_LOOKUP;
     new->d_wait = wq;
     hlist_bl_add_head_rcu(&new->d_u.d_in_lookup_hash, b);
     hlist_bl_unlock(b);
     return new;
 mismatch:
     spin_unlock(&dentry->d_lock);
     dput(dentry);
     goto retry;
 }
 EXPORT_SYMBOL(d_alloc_parallel);
 
 /*
  * - Unhash the dentry
  * - Retrieve and clear the waitqueue head in dentry
  * - Return the waitqueue head
  */
 static wait_queue_head_t *__d_lookup_unhash(struct dentry *dentry)
 {
     wait_queue_head_t *d_wait;
     struct hlist_bl_head *b;
 
     lockdep_assert_held(&dentry->d_lock);
 
     b = in_lookup_hash(dentry->d_parent, dentry->d_name.hash);
     hlist_bl_lock(b);
     dentry->d_flags &= ~DCACHE_PAR_LOOKUP;
     __hlist_bl_del(&dentry->d_u.d_in_lookup_hash);
     d_wait = dentry->d_wait;
     dentry->d_wait = NULL;
     hlist_bl_unlock(b);
     INIT_HLIST_NODE(&dentry->d_u.d_alias);
     INIT_LIST_HEAD(&dentry->d_lru);
     return d_wait;
 }
 
 void __d_lookup_unhash_wake(struct dentry *dentry)
 {
     spin_lock(&dentry->d_lock);
     wake_up_all(__d_lookup_unhash(dentry));
     spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(__d_lookup_unhash_wake);
 
 /* inode->i_lock held if inode is non-NULL */
 
 static inline void __d_add(struct dentry *dentry, struct inode *inode)
 {
     wait_queue_head_t *d_wait;
     struct inode *dir = NULL;
     unsigned n;
     spin_lock(&dentry->d_lock);
     if (unlikely(d_in_lookup(dentry))) {
         dir = dentry->d_parent->d_inode;
         n = start_dir_add(dir);
         d_wait = __d_lookup_unhash(dentry);
     }
     if (inode) {
         unsigned add_flags = d_flags_for_inode(inode);
         hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
         raw_write_seqcount_begin(&dentry->d_seq);
         __d_set_inode_and_type(dentry, inode, add_flags);
         raw_write_seqcount_end(&dentry->d_seq);
         fsnotify_update_flags(dentry);
     }
     __d_rehash(dentry);
     if (dir)
         end_dir_add(dir, n, d_wait);
     spin_unlock(&dentry->d_lock);
     if (inode)
         spin_unlock(&inode->i_lock);
 }
 
 /**
  * d_add - add dentry to hash queues
  * @entry: dentry to add
  * @inode: The inode to attach to this dentry
  *
  * This adds the entry to the hash queues and initializes @inode.
  * The entry was actually filled in earlier during d_alloc().
  */
 
 void d_add(struct dentry *entry, struct inode *inode)
 {
     if (inode) {
         security_d_instantiate(entry, inode);
         spin_lock(&inode->i_lock);
     }
     __d_add(entry, inode);
 }
 EXPORT_SYMBOL(d_add);
 
 /**
  * d_exact_alias - find and hash an exact unhashed alias
  * @entry: dentry to add
  * @inode: The inode to go with this dentry
  *
  * If an unhashed dentry with the same name/parent and desired
  * inode already exists, hash and return it.  Otherwise, return
  * NULL.
  *
  * Parent directory should be locked.
  */
 struct dentry *d_exact_alias(struct dentry *entry, struct inode *inode)
 {
     struct dentry *alias;
     unsigned int hash = entry->d_name.hash;
 
     spin_lock(&inode->i_lock);
     hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
         /*
          * Don't need alias->d_lock here, because aliases with
          * d_parent == entry->d_parent are not subject to name or
          * parent changes, because the parent inode i_mutex is held.
          */
         if (alias->d_name.hash != hash)
             continue;
         if (alias->d_parent != entry->d_parent)
             continue;
         if (!d_same_name(alias, entry->d_parent, &entry->d_name))
             continue;
         spin_lock(&alias->d_lock);
         if (!d_unhashed(alias)) {
             spin_unlock(&alias->d_lock);
             alias = NULL;
         } else {
             __dget_dlock(alias);
             __d_rehash(alias);
             spin_unlock(&alias->d_lock);
         }
         spin_unlock(&inode->i_lock);
         return alias;
     }
     spin_unlock(&inode->i_lock);
     return NULL;
 }
 EXPORT_SYMBOL(d_exact_alias);
 
 static void swap_names(struct dentry *dentry, struct dentry *target)
 {
     if (unlikely(dname_external(target))) {
         if (unlikely(dname_external(dentry))) {
             /*
              * Both external: swap the pointers
              */
             swap(target->d_name.name, dentry->d_name.name);
         } else {
             /*
              * dentry:internal, target:external.  Steal target's
              * storage and make target internal.
              */
             memcpy(target->d_iname, dentry->d_name.name,
                     dentry->d_name.len + 1);
             dentry->d_name.name = target->d_name.name;
             target->d_name.name = target->d_iname;
         }
     } else {
         if (unlikely(dname_external(dentry))) {
             /*
              * dentry:external, target:internal.  Give dentry's
              * storage to target and make dentry internal
              */
             memcpy(dentry->d_iname, target->d_name.name,
                     target->d_name.len + 1);
             target->d_name.name = dentry->d_name.name;
             dentry->d_name.name = dentry->d_iname;
         } else {
             /*
              * Both are internal.
              */
             unsigned int i;
             BUILD_BUG_ON(!IS_ALIGNED(DNAME_INLINE_LEN, sizeof(long)));
             for (i = 0; i < DNAME_INLINE_LEN / sizeof(long); i++) {
                 swap(((long *) &dentry->d_iname)[i],
                      ((long *) &target->d_iname)[i]);
             }
         }
     }
     swap(dentry->d_name.hash_len, target->d_name.hash_len);
 }
 
 static void copy_name(struct dentry *dentry, struct dentry *target)
 {
     struct external_name *old_name = NULL;
     if (unlikely(dname_external(dentry)))
         old_name = external_name(dentry);
     if (unlikely(dname_external(target))) {
         atomic_inc(&external_name(target)->u.count);
         dentry->d_name = target->d_name;
     } else {
         memcpy(dentry->d_iname, target->d_name.name,
                 target->d_name.len + 1);
         dentry->d_name.name = dentry->d_iname;
         dentry->d_name.hash_len = target->d_name.hash_len;
     }
     if (old_name && likely(atomic_dec_and_test(&old_name->u.count)))
         kfree_rcu(old_name, u.head);
 }
 
 /*
  * __d_move - move a dentry
  * @dentry: entry to move
  * @target: new dentry
  * @exchange: exchange the two dentries
  *
  * Update the dcache to reflect the move of a file name. Negative
  * dcache entries should not be moved in this way. Caller must hold
  * rename_lock, the i_mutex of the source and target directories,
  * and the sb->s_vfs_rename_mutex if they differ. See lock_rename().
  */
 static void __d_move(struct dentry *dentry, struct dentry *target,
              bool exchange)
 {
     struct dentry *old_parent, *p;
     wait_queue_head_t *d_wait;
     struct inode *dir = NULL;
     unsigned n;
 
     WARN_ON(!dentry->d_inode);
     if (WARN_ON(dentry == target))
         return;
 
     BUG_ON(d_ancestor(target, dentry));
     old_parent = dentry->d_parent;
     p = d_ancestor(old_parent, target);
     if (IS_ROOT(dentry)) {
         BUG_ON(p);
         spin_lock(&target->d_parent->d_lock);
     } else if (!p) {
         /* target is not a descendent of dentry->d_parent */
         spin_lock(&target->d_parent->d_lock);
         spin_lock_nested(&old_parent->d_lock, DENTRY_D_LOCK_NESTED);
     } else {
         BUG_ON(p == dentry);
         spin_lock(&old_parent->d_lock);
         if (p != target)
             spin_lock_nested(&target->d_parent->d_lock,
                     DENTRY_D_LOCK_NESTED);
     }
     spin_lock_nested(&dentry->d_lock, 2);
     spin_lock_nested(&target->d_lock, 3);
 
     if (unlikely(d_in_lookup(target))) {
         dir = target->d_parent->d_inode;
         n = start_dir_add(dir);
         d_wait = __d_lookup_unhash(target);
     }
 
     write_seqcount_begin(&dentry->d_seq);
     write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);
 
     /* unhash both */
     if (!d_unhashed(dentry))
         ___d_drop(dentry);
     if (!d_unhashed(target))
         ___d_drop(target);
 
     /* ... and switch them in the tree */
     dentry->d_parent = target->d_parent;
     if (!exchange) {
         copy_name(dentry, target);
         target->d_hash.pprev = NULL;
         dentry->d_parent->d_lockref.count++;
         if (dentry != old_parent) /* wasn't IS_ROOT */
             WARN_ON(!--old_parent->d_lockref.count);
     } else {
         target->d_parent = old_parent;
         swap_names(dentry, target);
         list_move(&target->d_child, &target->d_parent->d_subdirs);
         __d_rehash(target);
         fsnotify_update_flags(target);
     }
     list_move(&dentry->d_child, &dentry->d_parent->d_subdirs);
     __d_rehash(dentry);
     fsnotify_update_flags(dentry);
     fscrypt_handle_d_move(dentry);
 
     write_seqcount_end(&target->d_seq);
     write_seqcount_end(&dentry->d_seq);
 
     if (dir)
         end_dir_add(dir, n, d_wait);
 
     if (dentry->d_parent != old_parent)
         spin_unlock(&dentry->d_parent->d_lock);
     if (dentry != old_parent)
         spin_unlock(&old_parent->d_lock);
     spin_unlock(&target->d_lock);
     spin_unlock(&dentry->d_lock);
 }
 
 /*
  * d_move - move a dentry
  * @dentry: entry to move
  * @target: new dentry
  *
  * Update the dcache to reflect the move of a file name. Negative
  * dcache entries should not be moved in this way. See the locking
  * requirements for __d_move.
  */
 void d_move(struct dentry *dentry, struct dentry *target)
 {
     write_seqlock(&rename_lock);
     __d_move(dentry, target, false);
     write_sequnlock(&rename_lock);
 }
 EXPORT_SYMBOL(d_move);
 
 /*
  * d_exchange - exchange two dentries
  * @dentry1: first dentry
  * @dentry2: second dentry
  */
 void d_exchange(struct dentry *dentry1, struct dentry *dentry2)
 {
     write_seqlock(&rename_lock);
 
     WARN_ON(!dentry1->d_inode);
     WARN_ON(!dentry2->d_inode);
     WARN_ON(IS_ROOT(dentry1));
     WARN_ON(IS_ROOT(dentry2));
 
     __d_move(dentry1, dentry2, true);
 
     write_sequnlock(&rename_lock);
 }
 
 /**
  * d_ancestor - search for an ancestor
  * @p1: ancestor dentry
  * @p2: child dentry
  *
  * Returns the ancestor dentry of p2 which is a child of p1, if p1 is
  * an ancestor of p2, else NULL.
  */
 struct dentry *d_ancestor(struct dentry *p1, struct dentry *p2)
 {
     struct dentry *p;
 
     for (p = p2; !IS_ROOT(p); p = p->d_parent) {
         if (p->d_parent == p1)
             return p;
     }
     return NULL;
 }
 
 /*
  * This helper attempts to cope with remotely renamed directories
  *
  * It assumes that the caller is already holding
  * dentry->d_parent->d_inode->i_mutex, and rename_lock
  *
  * Note: If ever the locking in lock_rename() changes, then please
  * remember to update this too...
  */
 static int __d_unalias(struct inode *inode,
         struct dentry *dentry, struct dentry *alias)
 {
     struct mutex *m1 = NULL;
     struct rw_semaphore *m2 = NULL;
     int ret = -ESTALE;
 
     /* If alias and dentry share a parent, then no extra locks required */
     if (alias->d_parent == dentry->d_parent)
         goto out_unalias;
 
     /* See lock_rename() */
     if (!mutex_trylock(&dentry->d_sb->s_vfs_rename_mutex))
         goto out_err;
     m1 = &dentry->d_sb->s_vfs_rename_mutex;
     if (!inode_trylock_shared(alias->d_parent->d_inode))
         goto out_err;
     m2 = &alias->d_parent->d_inode->i_rwsem;
 out_unalias:
     __d_move(alias, dentry, false);
     ret = 0;
 out_err:
     if (m2)
         up_read(m2);
     if (m1)
         mutex_unlock(m1);
     return ret;
 }
 
 /**
  * d_splice_alias - splice a disconnected dentry into the tree if one exists
  * @inode:  the inode which may have a disconnected dentry
  * @dentry: a negative dentry which we want to point to the inode.
  *
  * If inode is a directory and has an IS_ROOT alias, then d_move that in
  * place of the given dentry and return it, else simply d_add the inode
  * to the dentry and return NULL.
  *
  * If a non-IS_ROOT directory is found, the filesystem is corrupt, and
  * we should error out: directories can't have multiple aliases.
  *
  * This is needed in the lookup routine of any filesystem that is exportable
  * (via knfsd) so that we can build dcache paths to directories effectively.
  *
  * If a dentry was found and moved, then it is returned.  Otherwise NULL
  * is returned.  This matches the expected return value of ->lookup.
  *
  * Cluster filesystems may call this function with a negative, hashed dentry.
  * In that case, we know that the inode will be a regular file, and also this
  * will only occur during atomic_open. So we need to check for the dentry
  * being already hashed only in the final case.
  */
 struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 {
     if (IS_ERR(inode))
         return ERR_CAST(inode);
 
     BUG_ON(!d_unhashed(dentry));
 
     if (!inode)
         goto out;
 
     security_d_instantiate(dentry, inode);
     spin_lock(&inode->i_lock);
     if (S_ISDIR(inode->i_mode)) {
         struct dentry *new = __d_find_any_alias(inode);
         if (unlikely(new)) {
             /* The reference to new ensures it remains an alias */
             spin_unlock(&inode->i_lock);
             write_seqlock(&rename_lock);
             if (unlikely(d_ancestor(new, dentry))) {
                 write_sequnlock(&rename_lock);
                 dput(new);
                 new = ERR_PTR(-ELOOP);
                 pr_warn_ratelimited(
                     "VFS: Lookup of '%s' in %s %s"
                     " would have caused loop\n",
                     dentry->d_name.name,
                     inode->i_sb->s_type->name,
                     inode->i_sb->s_id);
             } else if (!IS_ROOT(new)) {
                 struct dentry *old_parent = dget(new->d_parent);
                 int err = __d_unalias(inode, dentry, new);
                 write_sequnlock(&rename_lock);
                 if (err) {
                     dput(new);
                     new = ERR_PTR(err);
                 }
                 dput(old_parent);
             } else {
                 __d_move(new, dentry, false);
                 write_sequnlock(&rename_lock);
             }
             iput(inode);
             return new;
         }
     }
 out:
     __d_add(dentry, inode);
     return NULL;
 }
 EXPORT_SYMBOL(d_splice_alias);
 
 /*
  * Test whether new_dentry is a subdirectory of old_dentry.
  *
  * Trivially implemented using the dcache structure
  */
 
 /**
  * is_subdir - is new dentry a subdirectory of old_dentry
  * @new_dentry: new dentry
  * @old_dentry: old dentry
  *
  * Returns true if new_dentry is a subdirectory of the parent (at any depth).
  * Returns false otherwise.
  * Caller must ensure that "new_dentry" is pinned before calling is_subdir()
  */
   
 bool is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)
 {
     bool result;
     unsigned seq;
 
     if (new_dentry == old_dentry)
         return true;
 
     do {
         /* for restarting inner loop in case of seq retry */
         seq = read_seqbegin(&rename_lock);
         /*
          * Need rcu_readlock to protect against the d_parent trashing
          * due to d_move
          */
         rcu_read_lock();
         if (d_ancestor(old_dentry, new_dentry))
             result = true;
         else
             result = false;
         rcu_read_unlock();
     } while (read_seqretry(&rename_lock, seq));
 
     return result;
 }
 EXPORT_SYMBOL(is_subdir);
 
 static enum d_walk_ret d_genocide_kill(void *data, struct dentry *dentry)
 {
     struct dentry *root = data;
     if (dentry != root) {
         if (d_unhashed(dentry) || !dentry->d_inode)
             return D_WALK_SKIP;
 
         if (!(dentry->d_flags & DCACHE_GENOCIDE)) {
             dentry->d_flags |= DCACHE_GENOCIDE;
             dentry->d_lockref.count--;
         }
     }
     return D_WALK_CONTINUE;
 }
 
 void d_genocide(struct dentry *parent)
 {
     d_walk(parent, parent, d_genocide_kill);
 }
 
 EXPORT_SYMBOL(d_genocide);
 
 void d_tmpfile(struct dentry *dentry, struct inode *inode)
 {
     inode_dec_link_count(inode);
     BUG_ON(dentry->d_name.name != dentry->d_iname ||
         !hlist_unhashed(&dentry->d_u.d_alias) ||
         !d_unlinked(dentry));
     spin_lock(&dentry->d_parent->d_lock);
     spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
     dentry->d_name.len = sprintf(dentry->d_iname, "#%llu",
                 (unsigned long long)inode->i_ino);
     spin_unlock(&dentry->d_lock);
     spin_unlock(&dentry->d_parent->d_lock);
     d_instantiate(dentry, inode);
 }
 EXPORT_SYMBOL(d_tmpfile);
 
 static __initdata unsigned long dhash_entries;
 static int __init set_dhash_entries(char *str)
 {
     if (!str)
         return 0;
     dhash_entries = simple_strtoul(str, &str, 0);
     return 1;
 }
 __setup("dhash_entries=", set_dhash_entries);
 
 static void __init dcache_init_early(void)
 {
     /* If hashes are distributed across NUMA nodes, defer
      * hash allocation until vmalloc space is available.
      */
     if (hashdist)
         return;
 
     dentry_hashtable =
         alloc_large_system_hash("Dentry cache",
                     sizeof(struct hlist_bl_head),
                     dhash_entries,
                     13,
                     HASH_EARLY | HASH_ZERO,
                     &d_hash_shift,
                     NULL,
                     0,
                     0);
     d_hash_shift = 32 - d_hash_shift;
 }
 
 static void __init dcache_init(void)
 {
     /*
      * A constructor could be added for stable state like the lists,
      * but it is probably not worth it because of the cache nature
      * of the dcache.
      */
     dentry_cache = KMEM_CACHE_USERCOPY(dentry,
         SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_MEM_SPREAD|SLAB_ACCOUNT,
         d_iname);
 
     /* Hash may have been set up in dcache_init_early */
     if (!hashdist)
         return;
 
     dentry_hashtable =
         alloc_large_system_hash("Dentry cache",
                     sizeof(struct hlist_bl_head),
                     dhash_entries,
                     13,
                     HASH_ZERO,
                     &d_hash_shift,
                     NULL,
                     0,
                     0);
     d_hash_shift = 32 - d_hash_shift;
 }
 
 /* SLAB cache for __getname() consumers */
 struct kmem_cache *names_cachep __read_mostly;
 EXPORT_SYMBOL(names_cachep);
 
 void __init vfs_caches_init_early(void)
 {
     int i;
 
     for (i = 0; i < ARRAY_SIZE(in_lookup_hashtable); i++)
         INIT_HLIST_BL_HEAD(&in_lookup_hashtable[i]);
 
     dcache_init_early();
     inode_init_early();
 }
 
 void __init vfs_caches_init(void)
 {
     names_cachep = kmem_cache_create_usercopy("names_cache", PATH_MAX, 0,
             SLAB_HWCACHE_ALIGN|SLAB_PANIC, 0, PATH_MAX, NULL);
 
     dcache_init();
     inode_init();
     files_init();
     files_maxfiles_init();
     mnt_init();
     bdev_cache_init();
     chrdev_init();
 }