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 ====================
 The robust futex ABI
 ====================
 
 :Author: Started by Paul Jackson <pj@sgi.com>
 
 
 Robust_futexes provide a mechanism that is used in addition to normal
 futexes, for kernel assist of cleanup of held locks on task exit.
 
 The interesting data as to what futexes a thread is holding is kept on a
 linked list in user space, where it can be updated efficiently as locks
 are taken and dropped, without kernel intervention.  The only additional
 kernel intervention required for robust_futexes above and beyond what is
 required for futexes is:
 
  1) a one time call, per thread, to tell the kernel where its list of
     held robust_futexes begins, and
  2) internal kernel code at exit, to handle any listed locks held
     by the exiting thread.
 
 The existing normal futexes already provide a "Fast Userspace Locking"
 mechanism, which handles uncontested locking without needing a system
 call, and handles contested locking by maintaining a list of waiting
 threads in the kernel.  Options on the sys_futex(2) system call support
 waiting on a particular futex, and waking up the next waiter on a
 particular futex.
 
 For robust_futexes to work, the user code (typically in a library such
 as glibc linked with the application) has to manage and place the
 necessary list elements exactly as the kernel expects them.  If it fails
 to do so, then improperly listed locks will not be cleaned up on exit,
 probably causing deadlock or other such failure of the other threads
 waiting on the same locks.
 
 A thread that anticipates possibly using robust_futexes should first
 issue the system call::
 
     asmlinkage long
     sys_set_robust_list(struct robust_list_head __user *head, size_t len);
 
 The pointer 'head' points to a structure in the threads address space
 consisting of three words.  Each word is 32 bits on 32 bit arch's, or 64
 bits on 64 bit arch's, and local byte order.  Each thread should have
 its own thread private 'head'.
 
 If a thread is running in 32 bit compatibility mode on a 64 native arch
 kernel, then it can actually have two such structures - one using 32 bit
 words for 32 bit compatibility mode, and one using 64 bit words for 64
 bit native mode.  The kernel, if it is a 64 bit kernel supporting 32 bit
 compatibility mode, will attempt to process both lists on each task
 exit, if the corresponding sys_set_robust_list() call has been made to
 setup that list.
 
   The first word in the memory structure at 'head' contains a
   pointer to a single linked list of 'lock entries', one per lock,
   as described below.  If the list is empty, the pointer will point
   to itself, 'head'.  The last 'lock entry' points back to the 'head'.
 
   The second word, called 'offset', specifies the offset from the
   address of the associated 'lock entry', plus or minus, of what will
   be called the 'lock word', from that 'lock entry'.  The 'lock word'
   is always a 32 bit word, unlike the other words above.  The 'lock
   word' holds 2 flag bits in the upper 2 bits, and the thread id (TID)
   of the thread holding the lock in the bottom 30 bits.  See further
   below for a description of the flag bits.
 
   The third word, called 'list_op_pending', contains transient copy of
   the address of the 'lock entry', during list insertion and removal,
   and is needed to correctly resolve races should a thread exit while
   in the middle of a locking or unlocking operation.
 
 Each 'lock entry' on the single linked list starting at 'head' consists
 of just a single word, pointing to the next 'lock entry', or back to
 'head' if there are no more entries.  In addition, nearby to each 'lock
 entry', at an offset from the 'lock entry' specified by the 'offset'
 word, is one 'lock word'.
 
 The 'lock word' is always 32 bits, and is intended to be the same 32 bit
 lock variable used by the futex mechanism, in conjunction with
 robust_futexes.  The kernel will only be able to wakeup the next thread
 waiting for a lock on a threads exit if that next thread used the futex
 mechanism to register the address of that 'lock word' with the kernel.
 
 For each futex lock currently held by a thread, if it wants this
 robust_futex support for exit cleanup of that lock, it should have one
 'lock entry' on this list, with its associated 'lock word' at the
 specified 'offset'.  Should a thread die while holding any such locks,
 the kernel will walk this list, mark any such locks with a bit
 indicating their holder died, and wakeup the next thread waiting for
 that lock using the futex mechanism.
 
 When a thread has invoked the above system call to indicate it
 anticipates using robust_futexes, the kernel stores the passed in 'head'
 pointer for that task.  The task may retrieve that value later on by
 using the system call::
 
     asmlinkage long
     sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
                         size_t __user *len_ptr);
 
 It is anticipated that threads will use robust_futexes embedded in
 larger, user level locking structures, one per lock.  The kernel
 robust_futex mechanism doesn't care what else is in that structure, so
 long as the 'offset' to the 'lock word' is the same for all
 robust_futexes used by that thread.  The thread should link those locks
 it currently holds using the 'lock entry' pointers.  It may also have
 other links between the locks, such as the reverse side of a double
 linked list, but that doesn't matter to the kernel.
 
 By keeping its locks linked this way, on a list starting with a 'head'
 pointer known to the kernel, the kernel can provide to a thread the
 essential service available for robust_futexes, which is to help clean
 up locks held at the time of (a perhaps unexpectedly) exit.
 
 Actual locking and unlocking, during normal operations, is handled
 entirely by user level code in the contending threads, and by the
 existing futex mechanism to wait for, and wakeup, locks.  The kernels
 only essential involvement in robust_futexes is to remember where the
 list 'head' is, and to walk the list on thread exit, handling locks
 still held by the departing thread, as described below.
 
 There may exist thousands of futex lock structures in a threads shared
 memory, on various data structures, at a given point in time. Only those
 lock structures for locks currently held by that thread should be on
 that thread's robust_futex linked lock list a given time.
 
 A given futex lock structure in a user shared memory region may be held
 at different times by any of the threads with access to that region. The
 thread currently holding such a lock, if any, is marked with the threads
 TID in the lower 30 bits of the 'lock word'.
 
 When adding or removing a lock from its list of held locks, in order for
 the kernel to correctly handle lock cleanup regardless of when the task
 exits (perhaps it gets an unexpected signal 9 in the middle of
 manipulating this list), the user code must observe the following
 protocol on 'lock entry' insertion and removal:
 
 On insertion:
 
  1) set the 'list_op_pending' word to the address of the 'lock entry'
     to be inserted,
  2) acquire the futex lock,
  3) add the lock entry, with its thread id (TID) in the bottom 30 bits
     of the 'lock word', to the linked list starting at 'head', and
  4) clear the 'list_op_pending' word.
 
 On removal:
 
  1) set the 'list_op_pending' word to the address of the 'lock entry'
     to be removed,
  2) remove the lock entry for this lock from the 'head' list,
  3) release the futex lock, and
  4) clear the 'lock_op_pending' word.
 
 On exit, the kernel will consider the address stored in
 'list_op_pending' and the address of each 'lock word' found by walking
 the list starting at 'head'.  For each such address, if the bottom 30
 bits of the 'lock word' at offset 'offset' from that address equals the
 exiting threads TID, then the kernel will do two things:
 
  1) if bit 31 (0x80000000) is set in that word, then attempt a futex
     wakeup on that address, which will waken the next thread that has
     used to the futex mechanism to wait on that address, and
  2) atomically set  bit 30 (0x40000000) in the 'lock word'.
 
 In the above, bit 31 was set by futex waiters on that lock to indicate
 they were waiting, and bit 30 is set by the kernel to indicate that the
 lock owner died holding the lock.
 
 The kernel exit code will silently stop scanning the list further if at
 any point:
 
  1) the 'head' pointer or an subsequent linked list pointer
     is not a valid address of a user space word
  2) the calculated location of the 'lock word' (address plus
     'offset') is not the valid address of a 32 bit user space
     word
  3) if the list contains more than 1 million (subject to
     future kernel configuration changes) elements.
 
 When the kernel sees a list entry whose 'lock word' doesn't have the
 current threads TID in the lower 30 bits, it does nothing with that
 entry, and goes on to the next entry.

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