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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 entry'
- 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.
-
-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,
- 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 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 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.