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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+ */
 /*
  * Read-Copy Update mechanism for mutual exclusion (tree-based version)
  * Internal non-public definitions that provide either classic
  * or preemptible semantics.
  *
  * Copyright Red Hat, 2009
  * Copyright IBM Corporation, 2009
  *
  * Author: Ingo Molnar <mingo@elte.hu>
  *     Paul E. McKenney <paulmck@linux.ibm.com>
  */
 
 #include "../locking/rtmutex_common.h"
 
 static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
 {
     /*
      * In order to read the offloaded state of an rdp in a safe
      * and stable way and prevent from its value to be changed
      * under us, we must either hold the barrier mutex, the cpu
      * hotplug lock (read or write) or the nocb lock. Local
      * non-preemptible reads are also safe. NOCB kthreads and
      * timers have their own means of synchronization against the
      * offloaded state updaters.
      */
     RCU_LOCKDEP_WARN(
         !(lockdep_is_held(&rcu_state.barrier_mutex) ||
           (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
           rcu_lockdep_is_held_nocb(rdp) ||
           (rdp == this_cpu_ptr(&rcu_data) &&
            !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
           rcu_current_is_nocb_kthread(rdp)),
         "Unsafe read of RCU_NOCB offloaded state"
     );
 
     return rcu_segcblist_is_offloaded(&rdp->cblist);
 }
 
 /*
  * Check the RCU kernel configuration parameters and print informative
  * messages about anything out of the ordinary.
  */
 static void __init rcu_bootup_announce_oddness(void)
 {
     if (IS_ENABLED(CONFIG_RCU_TRACE))
         pr_info("\tRCU event tracing is enabled.\n");
     if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
         (!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))
         pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d.\n",
             RCU_FANOUT);
     if (rcu_fanout_exact)
         pr_info("\tHierarchical RCU autobalancing is disabled.\n");
     if (IS_ENABLED(CONFIG_PROVE_RCU))
         pr_info("\tRCU lockdep checking is enabled.\n");
     if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
         pr_info("\tRCU strict (and thus non-scalable) grace periods are enabled.\n");
     if (RCU_NUM_LVLS >= 4)
         pr_info("\tFour(or more)-level hierarchy is enabled.\n");
     if (RCU_FANOUT_LEAF != 16)
         pr_info("\tBuild-time adjustment of leaf fanout to %d.\n",
             RCU_FANOUT_LEAF);
     if (rcu_fanout_leaf != RCU_FANOUT_LEAF)
         pr_info("\tBoot-time adjustment of leaf fanout to %d.\n",
             rcu_fanout_leaf);
     if (nr_cpu_ids != NR_CPUS)
         pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%u.\n", NR_CPUS, nr_cpu_ids);
 #ifdef CONFIG_RCU_BOOST
     pr_info("\tRCU priority boosting: priority %d delay %d ms.\n",
         kthread_prio, CONFIG_RCU_BOOST_DELAY);
 #endif
     if (blimit != DEFAULT_RCU_BLIMIT)
         pr_info("\tBoot-time adjustment of callback invocation limit to %ld.\n", blimit);
     if (qhimark != DEFAULT_RCU_QHIMARK)
         pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
     if (qlowmark != DEFAULT_RCU_QLOMARK)
         pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
     if (qovld != DEFAULT_RCU_QOVLD)
         pr_info("\tBoot-time adjustment of callback overload level to %ld.\n", qovld);
     if (jiffies_till_first_fqs != ULONG_MAX)
         pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
     if (jiffies_till_next_fqs != ULONG_MAX)
         pr_info("\tBoot-time adjustment of subsequent FQS scan delay to %ld jiffies.\n", jiffies_till_next_fqs);
     if (jiffies_till_sched_qs != ULONG_MAX)
         pr_info("\tBoot-time adjustment of scheduler-enlistment delay to %ld jiffies.\n", jiffies_till_sched_qs);
     if (rcu_kick_kthreads)
         pr_info("\tKick kthreads if too-long grace period.\n");
     if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
         pr_info("\tRCU callback double-/use-after-free debug is enabled.\n");
     if (gp_preinit_delay)
         pr_info("\tRCU debug GP pre-init slowdown %d jiffies.\n", gp_preinit_delay);
     if (gp_init_delay)
         pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
     if (gp_cleanup_delay)
         pr_info("\tRCU debug GP cleanup slowdown %d jiffies.\n", gp_cleanup_delay);
     if (!use_softirq)
         pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
     if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
         pr_info("\tRCU debug extended QS entry/exit.\n");
     rcupdate_announce_bootup_oddness();
 }
 
 #ifdef CONFIG_PREEMPT_RCU
 
 static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
 static void rcu_read_unlock_special(struct task_struct *t);
 
 /*
  * Tell them what RCU they are running.
  */
 static void __init rcu_bootup_announce(void)
 {
     pr_info("Preemptible hierarchical RCU implementation.\n");
     rcu_bootup_announce_oddness();
 }
 
 /* Flags for rcu_preempt_ctxt_queue() decision table. */
 #define RCU_GP_TASKS    0x8
 #define RCU_EXP_TASKS   0x4
 #define RCU_GP_BLKD 0x2
 #define RCU_EXP_BLKD    0x1
 
 /*
  * Queues a task preempted within an RCU-preempt read-side critical
  * section into the appropriate location within the ->blkd_tasks list,
  * depending on the states of any ongoing normal and expedited grace
  * periods.  The ->gp_tasks pointer indicates which element the normal
  * grace period is waiting on (NULL if none), and the ->exp_tasks pointer
  * indicates which element the expedited grace period is waiting on (again,
  * NULL if none).  If a grace period is waiting on a given element in the
  * ->blkd_tasks list, it also waits on all subsequent elements.  Thus,
  * adding a task to the tail of the list blocks any grace period that is
  * already waiting on one of the elements.  In contrast, adding a task
  * to the head of the list won't block any grace period that is already
  * waiting on one of the elements.
  *
  * This queuing is imprecise, and can sometimes make an ongoing grace
  * period wait for a task that is not strictly speaking blocking it.
  * Given the choice, we needlessly block a normal grace period rather than
  * blocking an expedited grace period.
  *
  * Note that an endless sequence of expedited grace periods still cannot
  * indefinitely postpone a normal grace period.  Eventually, all of the
  * fixed number of preempted tasks blocking the normal grace period that are
  * not also blocking the expedited grace period will resume and complete
  * their RCU read-side critical sections.  At that point, the ->gp_tasks
  * pointer will equal the ->exp_tasks pointer, at which point the end of
  * the corresponding expedited grace period will also be the end of the
  * normal grace period.
  */
 static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
     __releases(rnp->lock) /* But leaves rrupts disabled. */
 {
     int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
              (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
              (rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
              (rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);
     struct task_struct *t = current;
 
     raw_lockdep_assert_held_rcu_node(rnp);
     WARN_ON_ONCE(rdp->mynode != rnp);
     WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
     /* RCU better not be waiting on newly onlined CPUs! */
     WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
              rdp->grpmask);
 
     /*
      * Decide where to queue the newly blocked task.  In theory,
      * this could be an if-statement.  In practice, when I tried
      * that, it was quite messy.
      */
     switch (blkd_state) {
     case 0:
     case                RCU_EXP_TASKS:
     case                RCU_EXP_TASKS + RCU_GP_BLKD:
     case RCU_GP_TASKS:
     case RCU_GP_TASKS + RCU_EXP_TASKS:
 
         /*
          * Blocking neither GP, or first task blocking the normal
          * GP but not blocking the already-waiting expedited GP.
          * Queue at the head of the list to avoid unnecessarily
          * blocking the already-waiting GPs.
          */
         list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
         break;
 
     case                                              RCU_EXP_BLKD:
     case                                RCU_GP_BLKD:
     case                                RCU_GP_BLKD + RCU_EXP_BLKD:
     case RCU_GP_TASKS +                               RCU_EXP_BLKD:
     case RCU_GP_TASKS +                 RCU_GP_BLKD + RCU_EXP_BLKD:
     case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
 
         /*
          * First task arriving that blocks either GP, or first task
          * arriving that blocks the expedited GP (with the normal
          * GP already waiting), or a task arriving that blocks
          * both GPs with both GPs already waiting.  Queue at the
          * tail of the list to avoid any GP waiting on any of the
          * already queued tasks that are not blocking it.
          */
         list_add_tail(&t->rcu_node_entry, &rnp->blkd_tasks);
         break;
 
     case                RCU_EXP_TASKS +               RCU_EXP_BLKD:
     case                RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
     case RCU_GP_TASKS + RCU_EXP_TASKS +               RCU_EXP_BLKD:
 
         /*
          * Second or subsequent task blocking the expedited GP.
          * The task either does not block the normal GP, or is the
          * first task blocking the normal GP.  Queue just after
          * the first task blocking the expedited GP.
          */
         list_add(&t->rcu_node_entry, rnp->exp_tasks);
         break;
 
     case RCU_GP_TASKS +                 RCU_GP_BLKD:
     case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD:
 
         /*
          * Second or subsequent task blocking the normal GP.
          * The task does not block the expedited GP. Queue just
          * after the first task blocking the normal GP.
          */
         list_add(&t->rcu_node_entry, rnp->gp_tasks);
         break;
 
     default:
 
         /* Yet another exercise in excessive paranoia. */
         WARN_ON_ONCE(1);
         break;
     }
 
     /*
      * We have now queued the task.  If it was the first one to
      * block either grace period, update the ->gp_tasks and/or
      * ->exp_tasks pointers, respectively, to reference the newly
      * blocked tasks.
      */
     if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
         WRITE_ONCE(rnp->gp_tasks, &t->rcu_node_entry);
         WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
     }
     if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
         WRITE_ONCE(rnp->exp_tasks, &t->rcu_node_entry);
     WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
              !(rnp->qsmask & rdp->grpmask));
     WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
              !(rnp->expmask & rdp->grpmask));
     raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */
 
     /*
      * Report the quiescent state for the expedited GP.  This expedited
      * GP should not be able to end until we report, so there should be
      * no need to check for a subsequent expedited GP.  (Though we are
      * still in a quiescent state in any case.)
      */
     if (blkd_state & RCU_EXP_BLKD && rdp->cpu_no_qs.b.exp)
         rcu_report_exp_rdp(rdp);
     else
         WARN_ON_ONCE(rdp->cpu_no_qs.b.exp);
 }
 
 /*
  * Record a preemptible-RCU quiescent state for the specified CPU.
  * Note that this does not necessarily mean that the task currently running
  * on the CPU is in a quiescent state:  Instead, it means that the current
  * grace period need not wait on any RCU read-side critical section that
  * starts later on this CPU.  It also means that if the current task is
  * in an RCU read-side critical section, it has already added itself to
  * some leaf rcu_node structure's ->blkd_tasks list.  In addition to the
  * current task, there might be any number of other tasks blocked while
  * in an RCU read-side critical section.
  *
  * Unlike non-preemptible-RCU, quiescent state reports for expedited
  * grace periods are handled separately via deferred quiescent states
  * and context switch events.
  *
  * Callers to this function must disable preemption.
  */
 static void rcu_qs(void)
 {
     RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!\n");
     if (__this_cpu_read(rcu_data.cpu_no_qs.b.norm)) {
         trace_rcu_grace_period(TPS("rcu_preempt"),
                        __this_cpu_read(rcu_data.gp_seq),
                        TPS("cpuqs"));
         __this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
         barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */
         WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, false);
     }
 }
 
 /*
  * We have entered the scheduler, and the current task might soon be
  * context-switched away from.  If this task is in an RCU read-side
  * critical section, we will no longer be able to rely on the CPU to
  * record that fact, so we enqueue the task on the blkd_tasks list.
  * The task will dequeue itself when it exits the outermost enclosing
  * RCU read-side critical section.  Therefore, the current grace period
  * cannot be permitted to complete until the blkd_tasks list entries
  * predating the current grace period drain, in other words, until
  * rnp->gp_tasks becomes NULL.
  *
  * Caller must disable interrupts.
  */
 void rcu_note_context_switch(bool preempt)
 {
     struct task_struct *t = current;
     struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
     struct rcu_node *rnp;
 
     trace_rcu_utilization(TPS("Start context switch"));
     lockdep_assert_irqs_disabled();
     WARN_ONCE(!preempt && rcu_preempt_depth() > 0, "Voluntary context switch within RCU read-side critical section!");
     if (rcu_preempt_depth() > 0 &&
         !t->rcu_read_unlock_special.b.blocked) {
 
         /* Possibly blocking in an RCU read-side critical section. */
         rnp = rdp->mynode;
         raw_spin_lock_rcu_node(rnp);
         t->rcu_read_unlock_special.b.blocked = true;
         t->rcu_blocked_node = rnp;
 
         /*
          * Verify the CPU's sanity, trace the preemption, and
          * then queue the task as required based on the states
          * of any ongoing and expedited grace periods.
          */
         WARN_ON_ONCE(!rcu_rdp_cpu_online(rdp));
         WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
         trace_rcu_preempt_task(rcu_state.name,
                        t->pid,
                        (rnp->qsmask & rdp->grpmask)
                        ? rnp->gp_seq
                        : rcu_seq_snap(&rnp->gp_seq));
         rcu_preempt_ctxt_queue(rnp, rdp);
     } else {
         rcu_preempt_deferred_qs(t);
     }
 
     /*
      * Either we were not in an RCU read-side critical section to
      * begin with, or we have now recorded that critical section
      * globally.  Either way, we can now note a quiescent state
      * for this CPU.  Again, if we were in an RCU read-side critical
      * section, and if that critical section was blocking the current
      * grace period, then the fact that the task has been enqueued
      * means that we continue to block the current grace period.
      */
     rcu_qs();
     if (rdp->cpu_no_qs.b.exp)
         rcu_report_exp_rdp(rdp);
     rcu_tasks_qs(current, preempt);
     trace_rcu_utilization(TPS("End context switch"));
 }
 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 
 /*
  * Check for preempted RCU readers blocking the current grace period
  * for the specified rcu_node structure.  If the caller needs a reliable
  * answer, it must hold the rcu_node's ->lock.
  */
 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 {
     return READ_ONCE(rnp->gp_tasks) != NULL;
 }
 
 /* limit value for ->rcu_read_lock_nesting. */
 #define RCU_NEST_PMAX (INT_MAX / 2)
 
 static void rcu_preempt_read_enter(void)
 {
     WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
 }
 
 static int rcu_preempt_read_exit(void)
 {
     int ret = READ_ONCE(current->rcu_read_lock_nesting) - 1;
 
     WRITE_ONCE(current->rcu_read_lock_nesting, ret);
     return ret;
 }
 
 static void rcu_preempt_depth_set(int val)
 {
     WRITE_ONCE(current->rcu_read_lock_nesting, val);
 }
 
 /*
  * Preemptible RCU implementation for rcu_read_lock().
  * Just increment ->rcu_read_lock_nesting, shared state will be updated
  * if we block.
  */
 void __rcu_read_lock(void)
 {
     rcu_preempt_read_enter();
     if (IS_ENABLED(CONFIG_PROVE_LOCKING))
         WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
     if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
         WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
     barrier();  /* critical section after entry code. */
 }
 EXPORT_SYMBOL_GPL(__rcu_read_lock);
 
 /*
  * Preemptible RCU implementation for rcu_read_unlock().
  * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
  * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
  * invoke rcu_read_unlock_special() to clean up after a context switch
  * in an RCU read-side critical section and other special cases.
  */
 void __rcu_read_unlock(void)
 {
     struct task_struct *t = current;
 
     barrier();  // critical section before exit code.
     if (rcu_preempt_read_exit() == 0) {
         barrier();  // critical-section exit before .s check.
         if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
             rcu_read_unlock_special(t);
     }
     if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
         int rrln = rcu_preempt_depth();
 
         WARN_ON_ONCE(rrln < 0 || rrln > RCU_NEST_PMAX);
     }
 }
 EXPORT_SYMBOL_GPL(__rcu_read_unlock);
 
 /*
  * Advance a ->blkd_tasks-list pointer to the next entry, instead
  * returning NULL if at the end of the list.
  */
 static struct list_head *rcu_next_node_entry(struct task_struct *t,
                          struct rcu_node *rnp)
 {
     struct list_head *np;
 
     np = t->rcu_node_entry.next;
     if (np == &rnp->blkd_tasks)
         np = NULL;
     return np;
 }
 
 /*
  * Return true if the specified rcu_node structure has tasks that were
  * preempted within an RCU read-side critical section.
  */
 static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
 {
     return !list_empty(&rnp->blkd_tasks);
 }
 
 /*
  * Report deferred quiescent states.  The deferral time can
  * be quite short, for example, in the case of the call from
  * rcu_read_unlock_special().
  */
 static notrace void
 rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 {
     bool empty_exp;
     bool empty_norm;
     bool empty_exp_now;
     struct list_head *np;
     bool drop_boost_mutex = false;
     struct rcu_data *rdp;
     struct rcu_node *rnp;
     union rcu_special special;
 
     /*
      * If RCU core is waiting for this CPU to exit its critical section,
      * report the fact that it has exited.  Because irqs are disabled,
      * t->rcu_read_unlock_special cannot change.
      */
     special = t->rcu_read_unlock_special;
     rdp = this_cpu_ptr(&rcu_data);
     if (!special.s && !rdp->cpu_no_qs.b.exp) {
         local_irq_restore(flags);
         return;
     }
     t->rcu_read_unlock_special.s = 0;
     if (special.b.need_qs) {
         if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {
             rdp->cpu_no_qs.b.norm = false;
             rcu_report_qs_rdp(rdp);
             udelay(rcu_unlock_delay);
         } else {
             rcu_qs();
         }
     }
 
     /*
      * Respond to a request by an expedited grace period for a
      * quiescent state from this CPU.  Note that requests from
      * tasks are handled when removing the task from the
      * blocked-tasks list below.
      */
     if (rdp->cpu_no_qs.b.exp)
         rcu_report_exp_rdp(rdp);
 
     /* Clean up if blocked during RCU read-side critical section. */
     if (special.b.blocked) {
 
         /*
          * Remove this task from the list it blocked on.  The task
          * now remains queued on the rcu_node corresponding to the
          * CPU it first blocked on, so there is no longer any need
          * to loop.  Retain a WARN_ON_ONCE() out of sheer paranoia.
          */
         rnp = t->rcu_blocked_node;
         raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
         WARN_ON_ONCE(rnp != t->rcu_blocked_node);
         WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
         empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
         WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
                  (!empty_norm || rnp->qsmask));
         empty_exp = sync_rcu_exp_done(rnp);
         smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
         np = rcu_next_node_entry(t, rnp);
         list_del_init(&t->rcu_node_entry);
         t->rcu_blocked_node = NULL;
         trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
                         rnp->gp_seq, t->pid);
         if (&t->rcu_node_entry == rnp->gp_tasks)
             WRITE_ONCE(rnp->gp_tasks, np);
         if (&t->rcu_node_entry == rnp->exp_tasks)
             WRITE_ONCE(rnp->exp_tasks, np);
         if (IS_ENABLED(CONFIG_RCU_BOOST)) {
             /* Snapshot ->boost_mtx ownership w/rnp->lock held. */
             drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx.rtmutex) == t;
             if (&t->rcu_node_entry == rnp->boost_tasks)
                 WRITE_ONCE(rnp->boost_tasks, np);
         }
 
         /*
          * If this was the last task on the current list, and if
          * we aren't waiting on any CPUs, report the quiescent state.
          * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
          * so we must take a snapshot of the expedited state.
          */
         empty_exp_now = sync_rcu_exp_done(rnp);
         if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
             trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
                              rnp->gp_seq,
                              0, rnp->qsmask,
                              rnp->level,
                              rnp->grplo,
                              rnp->grphi,
                              !!rnp->gp_tasks);
             rcu_report_unblock_qs_rnp(rnp, flags);
         } else {
             raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         }
 
         /*
          * If this was the last task on the expedited lists,
          * then we need to report up the rcu_node hierarchy.
          */
         if (!empty_exp && empty_exp_now)
             rcu_report_exp_rnp(rnp, true);
 
         /* Unboost if we were boosted. */
         if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
             rt_mutex_futex_unlock(&rnp->boost_mtx.rtmutex);
     } else {
         local_irq_restore(flags);
     }
 }
 
 /*
  * Is a deferred quiescent-state pending, and are we also not in
  * an RCU read-side critical section?  It is the caller's responsibility
  * to ensure it is otherwise safe to report any deferred quiescent
  * states.  The reason for this is that it is safe to report a
  * quiescent state during context switch even though preemption
  * is disabled.  This function cannot be expected to understand these
  * nuances, so the caller must handle them.
  */
 static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 {
     return (__this_cpu_read(rcu_data.cpu_no_qs.b.exp) ||
         READ_ONCE(t->rcu_read_unlock_special.s)) &&
            rcu_preempt_depth() == 0;
 }
 
 /*
  * Report a deferred quiescent state if needed and safe to do so.
  * As with rcu_preempt_need_deferred_qs(), "safe" involves only
  * not being in an RCU read-side critical section.  The caller must
  * evaluate safety in terms of interrupt, softirq, and preemption
  * disabling.
  */
 notrace void rcu_preempt_deferred_qs(struct task_struct *t)
 {
     unsigned long flags;
 
     if (!rcu_preempt_need_deferred_qs(t))
         return;
     local_irq_save(flags);
     rcu_preempt_deferred_qs_irqrestore(t, flags);
 }
 
 /*
  * Minimal handler to give the scheduler a chance to re-evaluate.
  */
 static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
 {
     struct rcu_data *rdp;
 
     rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
     rdp->defer_qs_iw_pending = false;
 }
 
 /*
  * Handle special cases during rcu_read_unlock(), such as needing to
  * notify RCU core processing or task having blocked during the RCU
  * read-side critical section.
  */
 static void rcu_read_unlock_special(struct task_struct *t)
 {
     unsigned long flags;
     bool irqs_were_disabled;
     bool preempt_bh_were_disabled =
             !!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
 
     /* NMI handlers cannot block and cannot safely manipulate state. */
     if (in_nmi())
         return;
 
     local_irq_save(flags);
     irqs_were_disabled = irqs_disabled_flags(flags);
     if (preempt_bh_were_disabled || irqs_were_disabled) {
         bool expboost; // Expedited GP in flight or possible boosting.
         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
         struct rcu_node *rnp = rdp->mynode;
 
         expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
                (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
                IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ||
                (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled &&
                 t->rcu_blocked_node);
         // Need to defer quiescent state until everything is enabled.
         if (use_softirq && (in_hardirq() || (expboost && !irqs_were_disabled))) {
             // Using softirq, safe to awaken, and either the
             // wakeup is free or there is either an expedited
             // GP in flight or a potential need to deboost.
             raise_softirq_irqoff(RCU_SOFTIRQ);
         } else {
             // Enabling BH or preempt does reschedule, so...
             // Also if no expediting and no possible deboosting,
             // slow is OK.  Plus nohz_full CPUs eventually get
             // tick enabled.
             set_tsk_need_resched(current);
             set_preempt_need_resched();
             if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
                 expboost && !rdp->defer_qs_iw_pending && cpu_online(rdp->cpu)) {
                 // Get scheduler to re-evaluate and call hooks.
                 // If !IRQ_WORK, FQS scan will eventually IPI.
                 if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
                     IS_ENABLED(CONFIG_PREEMPT_RT))
                     rdp->defer_qs_iw = IRQ_WORK_INIT_HARD(
                                 rcu_preempt_deferred_qs_handler);
                 else
                     init_irq_work(&rdp->defer_qs_iw,
                               rcu_preempt_deferred_qs_handler);
                 rdp->defer_qs_iw_pending = true;
                 irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
             }
         }
         local_irq_restore(flags);
         return;
     }
     rcu_preempt_deferred_qs_irqrestore(t, flags);
 }
 
 /*
  * Check that the list of blocked tasks for the newly completed grace
  * period is in fact empty.  It is a serious bug to complete a grace
  * period that still has RCU readers blocked!  This function must be
  * invoked -before- updating this rnp's ->gp_seq.
  *
  * Also, if there are blocked tasks on the list, they automatically
  * block the newly created grace period, so set up ->gp_tasks accordingly.
  */
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 {
     struct task_struct *t;
 
     RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
     raw_lockdep_assert_held_rcu_node(rnp);
     if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
         dump_blkd_tasks(rnp, 10);
     if (rcu_preempt_has_tasks(rnp) &&
         (rnp->qsmaskinit || rnp->wait_blkd_tasks)) {
         WRITE_ONCE(rnp->gp_tasks, rnp->blkd_tasks.next);
         t = container_of(rnp->gp_tasks, struct task_struct,
                  rcu_node_entry);
         trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
                         rnp->gp_seq, t->pid);
     }
     WARN_ON_ONCE(rnp->qsmask);
 }
 
 /*
  * Check for a quiescent state from the current CPU, including voluntary
  * context switches for Tasks RCU.  When a task blocks, the task is
  * recorded in the corresponding CPU's rcu_node structure, which is checked
  * elsewhere, hence this function need only check for quiescent states
  * related to the current CPU, not to those related to tasks.
  */
 static void rcu_flavor_sched_clock_irq(int user)
 {
     struct task_struct *t = current;
 
     lockdep_assert_irqs_disabled();
     if (user || rcu_is_cpu_rrupt_from_idle()) {
         rcu_note_voluntary_context_switch(current);
     }
     if (rcu_preempt_depth() > 0 ||
         (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
         /* No QS, force context switch if deferred. */
         if (rcu_preempt_need_deferred_qs(t)) {
             set_tsk_need_resched(t);
             set_preempt_need_resched();
         }
     } else if (rcu_preempt_need_deferred_qs(t)) {
         rcu_preempt_deferred_qs(t); /* Report deferred QS. */
         return;
     } else if (!WARN_ON_ONCE(rcu_preempt_depth())) {
         rcu_qs(); /* Report immediate QS. */
         return;
     }
 
     /* If GP is oldish, ask for help from rcu_read_unlock_special(). */
     if (rcu_preempt_depth() > 0 &&
         __this_cpu_read(rcu_data.core_needs_qs) &&
         __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
         !t->rcu_read_unlock_special.b.need_qs &&
         time_after(jiffies, rcu_state.gp_start + HZ))
         t->rcu_read_unlock_special.b.need_qs = true;
 }
 
 /*
  * Check for a task exiting while in a preemptible-RCU read-side
  * critical section, clean up if so.  No need to issue warnings, as
  * debug_check_no_locks_held() already does this if lockdep is enabled.
  * Besides, if this function does anything other than just immediately
  * return, there was a bug of some sort.  Spewing warnings from this
  * function is like as not to simply obscure important prior warnings.
  */
 void exit_rcu(void)
 {
     struct task_struct *t = current;
 
     if (unlikely(!list_empty(&current->rcu_node_entry))) {
         rcu_preempt_depth_set(1);
         barrier();
         WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
     } else if (unlikely(rcu_preempt_depth())) {
         rcu_preempt_depth_set(1);
     } else {
         return;
     }
     __rcu_read_unlock();
     rcu_preempt_deferred_qs(current);
 }
 
 /*
  * Dump the blocked-tasks state, but limit the list dump to the
  * specified number of elements.
  */
 static void
 dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
 {
     int cpu;
     int i;
     struct list_head *lhp;
     struct rcu_data *rdp;
     struct rcu_node *rnp1;
 
     raw_lockdep_assert_held_rcu_node(rnp);
     pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
         __func__, rnp->grplo, rnp->grphi, rnp->level,
         (long)READ_ONCE(rnp->gp_seq), (long)rnp->completedqs);
     for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
         pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
             __func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
     pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
         __func__, READ_ONCE(rnp->gp_tasks), data_race(rnp->boost_tasks),
         READ_ONCE(rnp->exp_tasks));
     pr_info("%s: ->blkd_tasks", __func__);
     i = 0;
     list_for_each(lhp, &rnp->blkd_tasks) {
         pr_cont(" %p", lhp);
         if (++i >= ncheck)
             break;
     }
     pr_cont("\n");
     for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
         rdp = per_cpu_ptr(&rcu_data, cpu);
         pr_info("\t%d: %c online: %ld(%d) offline: %ld(%d)\n",
             cpu, ".o"[rcu_rdp_cpu_online(rdp)],
             (long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_flags,
             (long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_flags);
     }
 }
 
 #else /* #ifdef CONFIG_PREEMPT_RCU */
 
 /*
  * If strict grace periods are enabled, and if the calling
  * __rcu_read_unlock() marks the beginning of a quiescent state, immediately
  * report that quiescent state and, if requested, spin for a bit.
  */
 void rcu_read_unlock_strict(void)
 {
     struct rcu_data *rdp;
 
     if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
         return;
     rdp = this_cpu_ptr(&rcu_data);
     rcu_report_qs_rdp(rdp);
     udelay(rcu_unlock_delay);
 }
 EXPORT_SYMBOL_GPL(rcu_read_unlock_strict);
 
 /*
  * Tell them what RCU they are running.
  */
 static void __init rcu_bootup_announce(void)
 {
     pr_info("Hierarchical RCU implementation.\n");
     rcu_bootup_announce_oddness();
 }
 
 /*
  * Note a quiescent state for PREEMPTION=n.  Because we do not need to know
  * how many quiescent states passed, just if there was at least one since
  * the start of the grace period, this just sets a flag.  The caller must
  * have disabled preemption.
  */
 static void rcu_qs(void)
 {
     RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!");
     if (!__this_cpu_read(rcu_data.cpu_no_qs.s))
         return;
     trace_rcu_grace_period(TPS("rcu_sched"),
                    __this_cpu_read(rcu_data.gp_seq), TPS("cpuqs"));
     __this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
     if (__this_cpu_read(rcu_data.cpu_no_qs.b.exp))
         rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
 }
 
 /*
  * Register an urgently needed quiescent state.  If there is an
  * emergency, invoke rcu_momentary_dyntick_idle() to do a heavy-weight
  * dyntick-idle quiescent state visible to other CPUs, which will in
  * some cases serve for expedited as well as normal grace periods.
  * Either way, register a lightweight quiescent state.
  */
 void rcu_all_qs(void)
 {
     unsigned long flags;
 
     if (!raw_cpu_read(rcu_data.rcu_urgent_qs))
         return;
     preempt_disable();
     /* Load rcu_urgent_qs before other flags. */
     if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
         preempt_enable();
         return;
     }
     this_cpu_write(rcu_data.rcu_urgent_qs, false);
     if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {
         local_irq_save(flags);
         rcu_momentary_dyntick_idle();
         local_irq_restore(flags);
     }
     rcu_qs();
     preempt_enable();
 }
 EXPORT_SYMBOL_GPL(rcu_all_qs);
 
 /*
  * Note a PREEMPTION=n context switch. The caller must have disabled interrupts.
  */
 void rcu_note_context_switch(bool preempt)
 {
     trace_rcu_utilization(TPS("Start context switch"));
     rcu_qs();
     /* Load rcu_urgent_qs before other flags. */
     if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs)))
         goto out;
     this_cpu_write(rcu_data.rcu_urgent_qs, false);
     if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))
         rcu_momentary_dyntick_idle();
 out:
     rcu_tasks_qs(current, preempt);
     trace_rcu_utilization(TPS("End context switch"));
 }
 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 
 /*
  * Because preemptible RCU does not exist, there are never any preempted
  * RCU readers.
  */
 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 {
     return 0;
 }
 
 /*
  * Because there is no preemptible RCU, there can be no readers blocked.
  */
 static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
 {
     return false;
 }
 
 /*
  * Because there is no preemptible RCU, there can be no deferred quiescent
  * states.
  */
 static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 {
     return false;
 }
 
 // Except that we do need to respond to a request by an expedited grace
 // period for a quiescent state from this CPU.  Note that requests from
 // tasks are handled when removing the task from the blocked-tasks list
 // below.
 notrace void rcu_preempt_deferred_qs(struct task_struct *t)
 {
     struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
     if (rdp->cpu_no_qs.b.exp)
         rcu_report_exp_rdp(rdp);
 }
 
 /*
  * Because there is no preemptible RCU, there can be no readers blocked,
  * so there is no need to check for blocked tasks.  So check only for
  * bogus qsmask values.
  */
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 {
     WARN_ON_ONCE(rnp->qsmask);
 }
 
 /*
  * Check to see if this CPU is in a non-context-switch quiescent state,
  * namely user mode and idle loop.
  */
 static void rcu_flavor_sched_clock_irq(int user)
 {
     if (user || rcu_is_cpu_rrupt_from_idle()) {
 
         /*
          * Get here if this CPU took its interrupt from user
          * mode or from the idle loop, and if this is not a
          * nested interrupt.  In this case, the CPU is in
          * a quiescent state, so note it.
          *
          * No memory barrier is required here because rcu_qs()
          * references only CPU-local variables that other CPUs
          * neither access nor modify, at least not while the
          * corresponding CPU is online.
          */
 
         rcu_qs();
     }
 }
 
 /*
  * Because preemptible RCU does not exist, tasks cannot possibly exit
  * while in preemptible RCU read-side critical sections.
  */
 void exit_rcu(void)
 {
 }
 
 /*
  * Dump the guaranteed-empty blocked-tasks state.  Trust but verify.
  */
 static void
 dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
 {
     WARN_ON_ONCE(!list_empty(&rnp->blkd_tasks));
 }
 
 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 
 /*
  * If boosting, set rcuc kthreads to realtime priority.
  */
 static void rcu_cpu_kthread_setup(unsigned int cpu)
 {
     struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 #ifdef CONFIG_RCU_BOOST
     struct sched_param sp;
 
     sp.sched_priority = kthread_prio;
     sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
 #endif /* #ifdef CONFIG_RCU_BOOST */
 
     WRITE_ONCE(rdp->rcuc_activity, jiffies);
 }
 
 static bool rcu_is_callbacks_nocb_kthread(struct rcu_data *rdp)
 {
 #ifdef CONFIG_RCU_NOCB_CPU
     return rdp->nocb_cb_kthread == current;
 #else
     return false;
 #endif
 }
 
 /*
  * Is the current CPU running the RCU-callbacks kthread?
  * Caller must have preemption disabled.
  */
 static bool rcu_is_callbacks_kthread(struct rcu_data *rdp)
 {
     return rdp->rcu_cpu_kthread_task == current ||
             rcu_is_callbacks_nocb_kthread(rdp);
 }
 
 #ifdef CONFIG_RCU_BOOST
 
 /*
  * Carry out RCU priority boosting on the task indicated by ->exp_tasks
  * or ->boost_tasks, advancing the pointer to the next task in the
  * ->blkd_tasks list.
  *
  * Note that irqs must be enabled: boosting the task can block.
  * Returns 1 if there are more tasks needing to be boosted.
  */
 static int rcu_boost(struct rcu_node *rnp)
 {
     unsigned long flags;
     struct task_struct *t;
     struct list_head *tb;
 
     if (READ_ONCE(rnp->exp_tasks) == NULL &&
         READ_ONCE(rnp->boost_tasks) == NULL)
         return 0;  /* Nothing left to boost. */
 
     raw_spin_lock_irqsave_rcu_node(rnp, flags);
 
     /*
      * Recheck under the lock: all tasks in need of boosting
      * might exit their RCU read-side critical sections on their own.
      */
     if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         return 0;
     }
 
     /*
      * Preferentially boost tasks blocking expedited grace periods.
      * This cannot starve the normal grace periods because a second
      * expedited grace period must boost all blocked tasks, including
      * those blocking the pre-existing normal grace period.
      */
     if (rnp->exp_tasks != NULL)
         tb = rnp->exp_tasks;
     else
         tb = rnp->boost_tasks;
 
     /*
      * We boost task t by manufacturing an rt_mutex that appears to
      * be held by task t.  We leave a pointer to that rt_mutex where
      * task t can find it, and task t will release the mutex when it
      * exits its outermost RCU read-side critical section.  Then
      * simply acquiring this artificial rt_mutex will boost task
      * t's priority.  (Thanks to tglx for suggesting this approach!)
      *
      * Note that task t must acquire rnp->lock to remove itself from
      * the ->blkd_tasks list, which it will do from exit() if from
      * nowhere else.  We therefore are guaranteed that task t will
      * stay around at least until we drop rnp->lock.  Note that
      * rnp->lock also resolves races between our priority boosting
      * and task t's exiting its outermost RCU read-side critical
      * section.
      */
     t = container_of(tb, struct task_struct, rcu_node_entry);
     rt_mutex_init_proxy_locked(&rnp->boost_mtx.rtmutex, t);
     raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
     /* Lock only for side effect: boosts task t's priority. */
     rt_mutex_lock(&rnp->boost_mtx);
     rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */
     rnp->n_boosts++;
 
     return READ_ONCE(rnp->exp_tasks) != NULL ||
            READ_ONCE(rnp->boost_tasks) != NULL;
 }
 
 /*
  * Priority-boosting kthread, one per leaf rcu_node.
  */
 static int rcu_boost_kthread(void *arg)
 {
     struct rcu_node *rnp = (struct rcu_node *)arg;
     int spincnt = 0;
     int more2boost;
 
     trace_rcu_utilization(TPS("Start boost kthread@init"));
     for (;;) {
         WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_WAITING);
         trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
         rcu_wait(READ_ONCE(rnp->boost_tasks) ||
              READ_ONCE(rnp->exp_tasks));
         trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
         WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_RUNNING);
         more2boost = rcu_boost(rnp);
         if (more2boost)
             spincnt++;
         else
             spincnt = 0;
         if (spincnt > 10) {
             WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_YIELDING);
             trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
             schedule_timeout_idle(2);
             trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
             spincnt = 0;
         }
     }
     /* NOTREACHED */
     trace_rcu_utilization(TPS("End boost kthread@notreached"));
     return 0;
 }
 
 /*
  * Check to see if it is time to start boosting RCU readers that are
  * blocking the current grace period, and, if so, tell the per-rcu_node
  * kthread to start boosting them.  If there is an expedited grace
  * period in progress, it is always time to boost.
  *
  * The caller must hold rnp->lock, which this function releases.
  * The ->boost_kthread_task is immortal, so we don't need to worry
  * about it going away.
  */
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
     __releases(rnp->lock)
 {
     raw_lockdep_assert_held_rcu_node(rnp);
     if (!rnp->boost_kthread_task ||
         (!rcu_preempt_blocked_readers_cgp(rnp) && !rnp->exp_tasks)) {
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         return;
     }
     if (rnp->exp_tasks != NULL ||
         (rnp->gp_tasks != NULL &&
          rnp->boost_tasks == NULL &&
          rnp->qsmask == 0 &&
          (!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld ||
           IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)))) {
         if (rnp->exp_tasks == NULL)
             WRITE_ONCE(rnp->boost_tasks, rnp->gp_tasks);
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         rcu_wake_cond(rnp->boost_kthread_task,
                   READ_ONCE(rnp->boost_kthread_status));
     } else {
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
     }
 }
 
 #define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
 
 /*
  * Do priority-boost accounting for the start of a new grace period.
  */
 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
 {
     rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
 }
 
 /*
  * Create an RCU-boost kthread for the specified node if one does not
  * already exist.  We only create this kthread for preemptible RCU.
  */
 static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 {
     unsigned long flags;
     int rnp_index = rnp - rcu_get_root();
     struct sched_param sp;
     struct task_struct *t;
 
     mutex_lock(&rnp->boost_kthread_mutex);
     if (rnp->boost_kthread_task || !rcu_scheduler_fully_active)
         goto out;
 
     t = kthread_create(rcu_boost_kthread, (void *)rnp,
                "rcub/%d", rnp_index);
     if (WARN_ON_ONCE(IS_ERR(t)))
         goto out;
 
     raw_spin_lock_irqsave_rcu_node(rnp, flags);
     rnp->boost_kthread_task = t;
     raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
     sp.sched_priority = kthread_prio;
     sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
     wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
 
  out:
     mutex_unlock(&rnp->boost_kthread_mutex);
 }
 
 /*
  * Set the per-rcu_node kthread's affinity to cover all CPUs that are
  * served by the rcu_node in question.  The CPU hotplug lock is still
  * held, so the value of rnp->qsmaskinit will be stable.
  *
  * We don't include outgoingcpu in the affinity set, use -1 if there is
  * no outgoing CPU.  If there are no CPUs left in the affinity set,
  * this function allows the kthread to execute on any CPU.
  */
 static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 {
     struct task_struct *t = rnp->boost_kthread_task;
     unsigned long mask = rcu_rnp_online_cpus(rnp);
     cpumask_var_t cm;
     int cpu;
 
     if (!t)
         return;
     if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
         return;
     mutex_lock(&rnp->boost_kthread_mutex);
     for_each_leaf_node_possible_cpu(rnp, cpu)
         if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
             cpu != outgoingcpu)
             cpumask_set_cpu(cpu, cm);
     cpumask_and(cm, cm, housekeeping_cpumask(HK_TYPE_RCU));
     if (cpumask_empty(cm))
         cpumask_copy(cm, housekeeping_cpumask(HK_TYPE_RCU));
     set_cpus_allowed_ptr(t, cm);
     mutex_unlock(&rnp->boost_kthread_mutex);
     free_cpumask_var(cm);
 }
 
 #else /* #ifdef CONFIG_RCU_BOOST */
 
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
     __releases(rnp->lock)
 {
     raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
 
 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
 {
 }
 
 static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 {
 }
 
 static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 {
 }
 
 #endif /* #else #ifdef CONFIG_RCU_BOOST */
 
 /*
  * Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
  * grace-period kthread will do force_quiescent_state() processing?
  * The idea is to avoid waking up RCU core processing on such a
  * CPU unless the grace period has extended for too long.
  *
  * This code relies on the fact that all NO_HZ_FULL CPUs are also
  * RCU_NOCB_CPU CPUs.
  */
 static bool rcu_nohz_full_cpu(void)
 {
 #ifdef CONFIG_NO_HZ_FULL
     if (tick_nohz_full_cpu(smp_processor_id()) &&
         (!rcu_gp_in_progress() ||
          time_before(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
         return true;
 #endif /* #ifdef CONFIG_NO_HZ_FULL */
     return false;
 }
 
 /*
  * Bind the RCU grace-period kthreads to the housekeeping CPU.
  */
 static void rcu_bind_gp_kthread(void)
 {
     if (!tick_nohz_full_enabled())
         return;
     housekeeping_affine(current, HK_TYPE_RCU);
 }

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