freebsd-dev/share/man/man3/pthread_mutex_unlock.3
Konstantin Belousov 2a339d9e3d Add implementation of robust mutexes, hopefully close enough to the
intention of the POSIX IEEE Std 1003.1TM-2008/Cor 1-2013.

A robust mutex is guaranteed to be cleared by the system upon either
thread or process owner termination while the mutex is held.  The next
mutex locker is then notified about inconsistent mutex state and can
execute (or abandon) corrective actions.

The patch mostly consists of small changes here and there, adding
neccessary checks for the inconsistent and abandoned conditions into
existing paths.  Additionally, the thread exit handler was extended to
iterate over the userspace-maintained list of owned robust mutexes,
unlocking and marking as terminated each of them.

The list of owned robust mutexes cannot be maintained atomically
synchronous with the mutex lock state (it is possible in kernel, but
is too expensive).  Instead, for the duration of lock or unlock
operation, the current mutex is remembered in a special slot that is
also checked by the kernel at thread termination.

Kernel must be aware about the per-thread location of the heads of
robust mutex lists and the current active mutex slot.  When a thread
touches a robust mutex for the first time, a new umtx op syscall is
issued which informs about location of lists heads.

The umtx sleep queues for PP and PI mutexes are split between
non-robust and robust.

Somewhat unrelated changes in the patch:
1. Style.
2. The fix for proper tdfind() call use in umtxq_sleep_pi() for shared
   pi mutexes.
3. Removal of the userspace struct pthread_mutex m_owner field.
4. The sysctl kern.ipc.umtx_vnode_persistent is added, which controls
   the lifetime of the shared mutex associated with a vnode' page.

Reviewed by:	jilles (previous version, supposedly the objection was fixed)
Discussed with:	brooks, Martin Simmons <martin@lispworks.com> (some aspects)
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
2016-05-17 09:56:22 +00:00

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Groff

.\" Copyright (c) 1997 Brian Cully <shmit@kublai.com>
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.Dd April 29, 2016
.Dt PTHREAD_MUTEX_UNLOCK 3
.Os
.Sh NAME
.Nm pthread_mutex_unlock
.Nd unlock a mutex
.Sh LIBRARY
.Lb libpthread
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_mutex_unlock "pthread_mutex_t *mutex"
.Sh DESCRIPTION
If the current thread holds the lock on
.Fa mutex ,
then the
.Fn pthread_mutex_unlock
function unlocks
.Fa mutex .
.Pp
If the argument pointed by the
.Fa mutex
is a robust mutex in the inconsistent state, and the call to
.Fn pthread_mutex_consistent
function was not done prior to unlocking, further locking attempts on
the mutex
.Fa mutex
are denied and locking functions return
.Er ENOTRECOVERABLE
error.
.Sh RETURN VALUES
If successful,
.Fn pthread_mutex_unlock
will return zero, otherwise an error number will be returned to
indicate the error.
.Sh ERRORS
The
.Fn pthread_mutex_unlock
function will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value specified by
.Fa mutex
is invalid.
.It Bq Er EPERM
The current thread does not hold a lock on
.Fa mutex .
.El
.Sh SEE ALSO
.Xr pthread_mutex_destroy 3 ,
.Xr pthread_mutex_init 3 ,
.Xr pthread_mutex_lock 3 ,
.Xr pthread_mutex_trylock 3
.Sh STANDARDS
The
.Fn pthread_mutex_unlock
function conforms to
.St -p1003.1-96 .