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Linux/Documentation/robust-futex-ABI.txt

  1 Started by Paul Jackson <pj@sgi.com>
  2 
  3 The robust futex ABI
  4 --------------------
  5 
  6 Robust_futexes provide a mechanism that is used in addition to normal
  7 futexes, for kernel assist of cleanup of held locks on task exit.
  8 
  9 The interesting data as to what futexes a thread is holding is kept on a
 10 linked list in user space, where it can be updated efficiently as locks
 11 are taken and dropped, without kernel intervention.  The only additional
 12 kernel intervention required for robust_futexes above and beyond what is
 13 required 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 
 20 The existing normal futexes already provide a "Fast Userspace Locking"
 21 mechanism, which handles uncontested locking without needing a system
 22 call, and handles contested locking by maintaining a list of waiting
 23 threads in the kernel.  Options on the sys_futex(2) system call support
 24 waiting on a particular futex, and waking up the next waiter on a
 25 particular futex.
 26 
 27 For robust_futexes to work, the user code (typically in a library such
 28 as glibc linked with the application) has to manage and place the
 29 necessary list elements exactly as the kernel expects them.  If it fails
 30 to do so, then improperly listed locks will not be cleaned up on exit,
 31 probably causing deadlock or other such failure of the other threads
 32 waiting on the same locks.
 33 
 34 A thread that anticipates possibly using robust_futexes should first
 35 issue the system call:
 36 
 37     asmlinkage long
 38     sys_set_robust_list(struct robust_list_head __user *head, size_t len);
 39 
 40 The pointer 'head' points to a structure in the threads address space
 41 consisting of three words.  Each word is 32 bits on 32 bit arch's, or 64
 42 bits on 64 bit arch's, and local byte order.  Each thread should have
 43 its own thread private 'head'.
 44 
 45 If a thread is running in 32 bit compatibility mode on a 64 native arch
 46 kernel, then it can actually have two such structures - one using 32 bit
 47 words for 32 bit compatibility mode, and one using 64 bit words for 64
 48 bit native mode.  The kernel, if it is a 64 bit kernel supporting 32 bit
 49 compatibility mode, will attempt to process both lists on each task
 50 exit, if the corresponding sys_set_robust_list() call has been made to
 51 setup 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 
 71 Each 'lock entry' on the single linked list starting at 'head' consists
 72 of 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
 74 entry', at an offset from the 'lock entry' specified by the 'offset'
 75 word, is one 'lock word'.
 76 
 77 The 'lock word' is always 32 bits, and is intended to be the same 32 bit
 78 lock variable used by the futex mechanism, in conjunction with
 79 robust_futexes.  The kernel will only be able to wakeup the next thread
 80 waiting for a lock on a threads exit if that next thread used the futex
 81 mechanism to register the address of that 'lock word' with the kernel.
 82 
 83 For each futex lock currently held by a thread, if it wants this
 84 robust_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
 86 specified 'offset'.  Should a thread die while holding any such locks,
 87 the kernel will walk this list, mark any such locks with a bit
 88 indicating their holder died, and wakeup the next thread waiting for
 89 that lock using the futex mechanism.
 90 
 91 When a thread has invoked the above system call to indicate it
 92 anticipates using robust_futexes, the kernel stores the passed in 'head'
 93 pointer for that task.  The task may retrieve that value later on by
 94 using 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 
100 It is anticipated that threads will use robust_futexes embedded in
101 larger, user level locking structures, one per lock.  The kernel
102 robust_futex mechanism doesn't care what else is in that structure, so
103 long as the 'offset' to the 'lock word' is the same for all
104 robust_futexes used by that thread.  The thread should link those locks
105 it currently holds using the 'lock entry' pointers.  It may also have
106 other links between the locks, such as the reverse side of a double
107 linked list, but that doesn't matter to the kernel.
108 
109 By keeping its locks linked this way, on a list starting with a 'head'
110 pointer known to the kernel, the kernel can provide to a thread the
111 essential service available for robust_futexes, which is to help clean
112 up locks held at the time of (a perhaps unexpectedly) exit.
113 
114 Actual locking and unlocking, during normal operations, is handled
115 entirely by user level code in the contending threads, and by the
116 existing futex mechanism to wait for, and wakeup, locks.  The kernels
117 only essential involvement in robust_futexes is to remember where the
118 list 'head' is, and to walk the list on thread exit, handling locks
119 still held by the departing thread, as described below.
120 
121 There may exist thousands of futex lock structures in a threads shared
122 memory, on various data structures, at a given point in time. Only those
123 lock structures for locks currently held by that thread should be on
124 that thread's robust_futex linked lock list a given time.
125 
126 A given futex lock structure in a user shared memory region may be held
127 at different times by any of the threads with access to that region. The
128 thread currently holding such a lock, if any, is marked with the threads
129 TID in the lower 29 bits of the 'lock word'.
130 
131 When adding or removing a lock from its list of held locks, in order for
132 the kernel to correctly handle lock cleanup regardless of when the task
133 exits (perhaps it gets an unexpected signal 9 in the middle of
134 manipulating this list), the user code must observe the following
135 protocol on 'lock entry' insertion and removal:
136 
137 On 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 
145 On 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  3) release the futex lock, and
150  4) clear the 'lock_op_pending' word.
151 
152 On exit, the kernel will consider the address stored in
153 'list_op_pending' and the address of each 'lock word' found by walking
154 the list starting at 'head'.  For each such address, if the bottom 29
155 bits of the 'lock word' at offset 'offset' from that address equals the
156 exiting 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 
163 In the above, bit 31 was set by futex waiters on that lock to indicate
164 they were waiting, and bit 30 is set by the kernel to indicate that the
165 lock owner died holding the lock.
166 
167 The kernel exit code will silently stop scanning the list further if at
168 any 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 
178 When the kernel sees a list entry whose 'lock word' doesn't have the
179 current threads TID in the lower 29 bits, it does nothing with that
180 entry, and goes on to the next entry.
181 
182 Bit 29 (0x20000000) of the 'lock word' is reserved for future use.

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