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