linux/Documentation/robust-futex-ABI.txt
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   1Started by Paul Jackson <pj@sgi.com>
   2
   3The robust futex ABI
   4--------------------
   5
   6Robust_futexes provide a mechanism that is used in addition to normal
   7futexes, for kernel assist of cleanup of held locks on task exit.
   8
   9The interesting data as to what futexes a thread is holding is kept on a
  10linked list in user space, where it can be updated efficiently as locks
  11are taken and dropped, without kernel intervention.  The only additional
  12kernel intervention required for robust_futexes above and beyond what is
  13required for futexes is:
  14
  15 1) a one time call, per thread, to tell the kernel where its list of
  16    held robust_futexes begins, and
  17 2) internal kernel code at exit, to handle any listed locks held
  18    by the exiting thread.
  19
  20The existing normal futexes already provide a "Fast Userspace Locking"
  21mechanism, which handles uncontested locking without needing a system
  22call, and handles contested locking by maintaining a list of waiting
  23threads in the kernel.  Options on the sys_futex(2) system call support
  24waiting on a particular futex, and waking up the next waiter on a
  25particular futex.
  26
  27For robust_futexes to work, the user code (typically in a library such
  28as glibc linked with the application) has to manage and place the
  29necessary list elements exactly as the kernel expects them.  If it fails
  30to do so, then improperly listed locks will not be cleaned up on exit,
  31probably causing deadlock or other such failure of the other threads
  32waiting on the same locks.
  33
  34A thread that anticipates possibly using robust_futexes should first
  35issue the system call:
  36
  37    asmlinkage long
  38    sys_set_robust_list(struct robust_list_head __user *head, size_t len);
  39
  40The pointer 'head' points to a structure in the threads address space
  41consisting of three words.  Each word is 32 bits on 32 bit arch's, or 64
  42bits on 64 bit arch's, and local byte order.  Each thread should have
  43its own thread private 'head'.
  44
  45If a thread is running in 32 bit compatibility mode on a 64 native arch
  46kernel, then it can actually have two such structures - one using 32 bit
  47words for 32 bit compatibility mode, and one using 64 bit words for 64
  48bit native mode.  The kernel, if it is a 64 bit kernel supporting 32 bit
  49compatibility mode, will attempt to process both lists on each task
  50exit, if the corresponding sys_set_robust_list() call has been made to
  51setup that list.
  52
  53  The first word in the memory structure at 'head' contains a
  54  pointer to a single linked list of 'lock entries', one per lock,
  55  as described below.  If the list is empty, the pointer will point
  56  to itself, 'head'.  The last 'lock entry' points back to the 'head'.
  57
  58  The second word, called 'offset', specifies the offset from the
  59  address of the associated 'lock entry', plus or minus, of what will
  60  be called the 'lock word', from that 'lock entry'.  The 'lock word'
  61  is always a 32 bit word, unlike the other words above.  The 'lock
  62  word' holds 3 flag bits in the upper 3 bits, and the thread id (TID)
  63  of the thread holding the lock in the bottom 29 bits.  See further
  64  below for a description of the flag bits.
  65
  66  The third word, called 'list_op_pending', contains transient copy of
  67  the address of the 'lock entry', during list insertion and removal,
  68  and is needed to correctly resolve races should a thread exit while
  69  in the middle of a locking or unlocking operation.
  70
  71Each 'lock entry' on the single linked list starting at 'head' consists
  72of just a single word, pointing to the next 'lock entry', or back to
  73'head' if there are no more entries.  In addition, nearby to each 'lock
  74entry', at an offset from the 'lock entry' specified by the 'offset'
  75word, is one 'lock word'.
  76
  77The 'lock word' is always 32 bits, and is intended to be the same 32 bit
  78lock variable used by the futex mechanism, in conjunction with
  79robust_futexes.  The kernel will only be able to wakeup the next thread
  80waiting for a lock on a threads exit if that next thread used the futex
  81mechanism to register the address of that 'lock word' with the kernel.
  82
  83For each futex lock currently held by a thread, if it wants this
  84robust_futex support for exit cleanup of that lock, it should have one
  85'lock entry' on this list, with its associated 'lock word' at the
  86specified 'offset'.  Should a thread die while holding any such locks,
  87the kernel will walk this list, mark any such locks with a bit
  88indicating their holder died, and wakeup the next thread waiting for
  89that lock using the futex mechanism.
  90
  91When a thread has invoked the above system call to indicate it
  92anticipates using robust_futexes, the kernel stores the passed in 'head'
  93pointer for that task.  The task may retrieve that value later on by
  94using the system call:
  95
  96    asmlinkage long
  97    sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
  98                        size_t __user *len_ptr);
  99
 100It is anticipated that threads will use robust_futexes embedded in
 101larger, user level locking structures, one per lock.  The kernel
 102robust_futex mechanism doesn't care what else is in that structure, so
 103long as the 'offset' to the 'lock word' is the same for all
 104robust_futexes used by that thread.  The thread should link those locks
 105it currently holds using the 'lock entry' pointers.  It may also have
 106other links between the locks, such as the reverse side of a double
 107linked list, but that doesn't matter to the kernel.
 108
 109By keeping its locks linked this way, on a list starting with a 'head'
 110pointer known to the kernel, the kernel can provide to a thread the
 111essential service available for robust_futexes, which is to help clean
 112up locks held at the time of (a perhaps unexpectedly) exit.
 113
 114Actual locking and unlocking, during normal operations, is handled
 115entirely by user level code in the contending threads, and by the
 116existing futex mechanism to wait for, and wakeup, locks.  The kernels
 117only essential involvement in robust_futexes is to remember where the
 118list 'head' is, and to walk the list on thread exit, handling locks
 119still held by the departing thread, as described below.
 120
 121There may exist thousands of futex lock structures in a threads shared
 122memory, on various data structures, at a given point in time. Only those
 123lock structures for locks currently held by that thread should be on
 124that thread's robust_futex linked lock list a given time.
 125
 126A given futex lock structure in a user shared memory region may be held
 127at different times by any of the threads with access to that region. The
 128thread currently holding such a lock, if any, is marked with the threads
 129TID in the lower 29 bits of the 'lock word'.
 130
 131When adding or removing a lock from its list of held locks, in order for
 132the kernel to correctly handle lock cleanup regardless of when the task
 133exits (perhaps it gets an unexpected signal 9 in the middle of
 134manipulating this list), the user code must observe the following
 135protocol on 'lock entry' insertion and removal:
 136
 137On insertion:
 138 1) set the 'list_op_pending' word to the address of the 'lock entry'
 139    to be inserted,
 140 2) acquire the futex lock,
 141 3) add the lock entry, with its thread id (TID) in the bottom 29 bits
 142    of the 'lock word', to the linked list starting at 'head', and
 143 4) clear the 'list_op_pending' word.
 144
 145On removal:
 146 1) set the 'list_op_pending' word to the address of the 'lock entry'
 147    to be removed,
 148 2) remove the lock entry for this lock from the 'head' list,
 149 2) release the futex lock, and
 150 2) clear the 'lock_op_pending' word.
 151
 152On exit, the kernel will consider the address stored in
 153'list_op_pending' and the address of each 'lock word' found by walking
 154the list starting at 'head'.  For each such address, if the bottom 29
 155bits of the 'lock word' at offset 'offset' from that address equals the
 156exiting threads TID, then the kernel will do two things:
 157
 158 1) if bit 31 (0x80000000) is set in that word, then attempt a futex
 159    wakeup on that address, which will waken the next thread that has
 160    used to the futex mechanism to wait on that address, and
 161 2) atomically set  bit 30 (0x40000000) in the 'lock word'.
 162
 163In the above, bit 31 was set by futex waiters on that lock to indicate
 164they were waiting, and bit 30 is set by the kernel to indicate that the
 165lock owner died holding the lock.
 166
 167The kernel exit code will silently stop scanning the list further if at
 168any point:
 169
 170 1) the 'head' pointer or an subsequent linked list pointer
 171    is not a valid address of a user space word
 172 2) the calculated location of the 'lock word' (address plus
 173    'offset') is not the valid address of a 32 bit user space
 174    word
 175 3) if the list contains more than 1 million (subject to
 176    future kernel configuration changes) elements.
 177
 178When the kernel sees a list entry whose 'lock word' doesn't have the
 179current threads TID in the lower 29 bits, it does nothing with that
 180entry, and goes on to the next entry.
 181
 182Bit 29 (0x20000000) of the 'lock word' is reserved for future use.
 183
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