[PATCH] lightweight robust futexes: docs

Add robust-futex documentation.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 184 insertions, 0 deletions

diff --git a/Documentation/robust-futex-ABI.txt b/Documentation/robust-futex-ABI.txt
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+Started by Paul Jackson <pj@sgi.com>
+
+The robust futex ABI
+--------------------
+
+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 3 flag bits in the upper 3 bits, and the thread id (TID)
+  of the thread holding the lock in the bottom 29 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 29 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 word'
+    to be inserted,
+ 2) acquire the futex lock,
+ 3) add the lock entry, with its thread id (TID) in the bottom 29 bits
+    of the 'lock word', to the linked list starting at 'head', and
+ 4) clear the 'list_op_pending' word.
+
+        XXX I am particularly unsure of the following -pj XXX
+
+On removal:
+ 1) set the 'list_op_pending' word to the address of the 'lock word'
+    to be removed,
+ 2) remove the lock entry for this lock from the 'head' list,
+ 2) release the futex lock, and
+ 2) 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 29
+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 valud 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 29 bits, it does nothing with that
+entry, and goes on to the next entry.
+
+Bit 29 (0x20000000) of the 'lock word' is reserved for future use.