.\" .\" Copyright (C) 2002 Chad David . All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice(s), this list of conditions and the following disclaimer as .\" the first lines of this file unmodified other than the possible .\" addition of one or more copyright notices. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice(s), this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY .\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE .\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY .\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES .\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR .\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER .\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH .\" DAMAGE. .\" .\" $FreeBSD: head/share/man/man9/lock.9 301590 2016-06-08 09:19:47Z trasz $ .\" .Dd November 2, 2014 .Dt LOCK 9 .Os .Sh NAME .Nm lockinit , .Nm lockdestroy , .Nm lockmgr , .Nm lockmgr_args , .Nm lockmgr_args_rw , .Nm lockmgr_disown , .Nm lockmgr_printinfo , .Nm lockmgr_recursed , .Nm lockmgr_rw , .Nm lockmgr_waiters , .Nm lockstatus , .Nm lockmgr_assert .Nd "lockmgr family of functions" .Sh SYNOPSIS .In sys/types.h .In sys/lock.h .In sys/lockmgr.h .Ft void .Fn lockinit "struct lock *lkp" "int prio" "const char *wmesg" "int timo" "int flags" .Ft void .Fn lockdestroy "struct lock *lkp" .Ft int .Fn lockmgr "struct lock *lkp" "u_int flags" "struct mtx *ilk" .Ft int .Fn lockmgr_args "struct lock *lkp" "u_int flags" "struct mtx *ilk" "const char *wmesg" "int prio" "int timo" .Ft int .Fn lockmgr_args_rw "struct lock *lkp" "u_int flags" "struct rwlock *ilk" "const char *wmesg" "int prio" "int timo" .Ft void .Fn lockmgr_disown "struct lock *lkp" .Ft void .Fn lockmgr_printinfo "const struct lock *lkp" .Ft int .Fn lockmgr_recursed "const struct lock *lkp" .Ft int .Fn lockmgr_rw "struct lock *lkp" "u_int flags" "struct rwlock *ilk" .Ft int .Fn lockmgr_waiters "const struct lock *lkp" .Ft int .Fn lockstatus "const struct lock *lkp" .Pp .Cd "options INVARIANTS" .Cd "options INVARIANT_SUPPORT" .Ft void .Fn lockmgr_assert "const struct lock *lkp" "int what" .Sh DESCRIPTION The .Fn lockinit function is used to initialize a lock. It must be called before any operation can be performed on a lock. Its arguments are: .Bl -tag -width ".Fa wmesg" .It Fa lkp A pointer to the lock to initialize. .It Fa prio The priority passed to .Xr sleep 9 . .It Fa wmesg The lock message. This is used for both debugging output and .Xr sleep 9 . .It Fa timo The timeout value passed to .Xr sleep 9 . .It Fa flags The flags the lock is to be initialized with: .Bl -tag -width ".Dv LK_CANRECURSE" .It Dv LK_ADAPTIVE Enable adaptive spinning for this lock if the kernel is compiled with the ADAPTIVE_LOCKMGRS option. .It Dv LK_CANRECURSE Allow recursive exclusive locks. .It Dv LK_NOPROFILE Disable lock profiling for this lock. .It Dv LK_NOSHARE Allow exclusive locks only. .It Dv LK_NOWITNESS Instruct .Xr witness 4 to ignore this lock. .It Dv LK_NODUP .Xr witness 4 should log messages about duplicate locks being acquired. .It Dv LK_QUIET Disable .Xr ktr 4 logging for this lock. .It Dv LK_TIMELOCK Use .Fa timo during a sleep; otherwise, 0 is used. .El .El .Pp The .Fn lockdestroy function is used to destroy a lock, and while it is called in a number of places in the kernel, it currently does nothing. .Pp The .Fn lockmgr and .Fn lockmgr_rw functions handle general locking functionality within the kernel, including support for shared and exclusive locks, and recursion. .Fn lockmgr and .Fn lockmgr_rw are also able to upgrade and downgrade locks. .Pp Their arguments are: .Bl -tag -width ".Fa flags" .It Fa lkp A pointer to the lock to manipulate. .It Fa flags Flags indicating what action is to be taken. .Bl -tag -width ".Dv LK_NODDLKTREAT" .It Dv LK_SHARED Acquire a shared lock. If an exclusive lock is currently held, .Dv EDEADLK will be returned. .It Dv LK_EXCLUSIVE Acquire an exclusive lock. If an exclusive lock is already held, and .Dv LK_CANRECURSE is not set, the system will .Xr panic 9 . .It Dv LK_DOWNGRADE Downgrade exclusive lock to a shared lock. Downgrading a shared lock is not permitted. If an exclusive lock has been recursed, the system will .Xr panic 9 . .It Dv LK_UPGRADE Upgrade a shared lock to an exclusive lock. If this call fails, the shared lock is lost, even if the .Dv LK_NOWAIT flag is specified. During the upgrade, the shared lock could be temporarily dropped. Attempts to upgrade an exclusive lock will cause a .Xr panic 9 . .It Dv LK_TRYUPGRADE Try to upgrade a shared lock to an exclusive lock. The failure to upgrade does not result in the dropping of the shared lock ownership. .It Dv LK_RELEASE Release the lock. Releasing a lock that is not held can cause a .Xr panic 9 . .It Dv LK_DRAIN Wait for all activity on the lock to end, then mark it decommissioned. This is used before freeing a lock that is part of a piece of memory that is about to be freed. (As documented in .In sys/lockmgr.h . ) .It Dv LK_SLEEPFAIL Fail if operation has slept. .It Dv LK_NOWAIT Do not allow the call to sleep. This can be used to test the lock. .It Dv LK_NOWITNESS Skip the .Xr witness 4 checks for this instance. .It Dv LK_CANRECURSE Allow recursion on an exclusive lock. For every lock there must be a release. .It Dv LK_INTERLOCK Unlock the interlock (which should be locked already). .It Dv LK_NODDLKTREAT Normally, .Fn lockmgr postpones serving further shared requests for shared-locked lock if there is exclusive waiter, to avoid exclusive lock starvation. But, if the thread requesting the shared lock already owns a shared lockmgr lock, the request is granted even in presence of the parallel exclusive lock request, which is done to avoid deadlocks with recursive shared acquisition. .Pp The .Dv LK_NODDLKTREAT flag can only be used by code which requests shared non-recursive lock. The flag allows exclusive requests to preempt the current shared request even if the current thread owns shared locks. This is safe since shared lock is guaranteed to not recurse, and is used when thread is known to held unrelated shared locks, to not cause unnecessary starvation. An example is .Dv vp locking in VFS .Xr lookup 9 , when .Dv dvp is already locked. .El .It Fa ilk An interlock mutex for controlling group access to the lock. If .Dv LK_INTERLOCK is specified, .Fn lockmgr and .Fn lockmgr_rw assume .Fa ilk is currently owned and not recursed, and will return it unlocked. See .Xr mtx_assert 9 . .El .Pp The .Fn lockmgr_args and .Fn lockmgr_args_rw function work like .Fn lockmgr and .Fn lockmgr_rw but accepting a .Fa wmesg , .Fa timo and .Fa prio on a per-instance basis. The specified values will override the default ones, but this can still be used passing, respectively, .Dv LK_WMESG_DEFAULT , .Dv LK_PRIO_DEFAULT and .Dv LK_TIMO_DEFAULT . .Pp The .Fn lockmgr_disown function switches the owner from the current thread to be .Dv LK_KERNPROC , if the lock is already held. .Pp The .Fn lockmgr_printinfo function prints debugging information about the lock. It is used primarily by .Xr VOP_PRINT 9 functions. .Pp The .Fn lockmgr_recursed function returns true if the lock is recursed, 0 otherwise. .Pp The .Fn lockmgr_waiters function returns true if the lock has waiters, 0 otherwise. .Pp The .Fn lockstatus function returns the status of the lock in relation to the current thread. .Pp When compiled with .Cd "options INVARIANTS" and .Cd "options INVARIANT_SUPPORT" , the .Fn lockmgr_assert function tests .Fa lkp for the assertions specified in .Fa what , and panics if they are not met. One of the following assertions must be specified: .Bl -tag -width ".Dv KA_UNLOCKED" .It Dv KA_LOCKED Assert that the current thread has either a shared or an exclusive lock on the .Vt lkp lock pointed to by the first argument. .It Dv KA_SLOCKED Assert that the current thread has a shared lock on the .Vt lkp lock pointed to by the first argument. .It Dv KA_XLOCKED Assert that the current thread has an exclusive lock on the .Vt lkp lock pointed to by the first argument. .It Dv KA_UNLOCKED Assert that the current thread has no lock on the .Vt lkp lock pointed to by the first argument. .El .Pp In addition, one of the following optional assertions can be used with either an .Dv KA_LOCKED , .Dv KA_SLOCKED , or .Dv KA_XLOCKED assertion: .Bl -tag -width ".Dv KA_NOTRECURSED" .It Dv KA_RECURSED Assert that the current thread has a recursed lock on .Fa lkp . .It Dv KA_NOTRECURSED Assert that the current thread does not have a recursed lock on .Fa lkp . .El .Sh RETURN VALUES The .Fn lockmgr and .Fn lockmgr_rw functions return 0 on success and non-zero on failure. .Pp The .Fn lockstatus function returns: .Bl -tag -width ".Dv LK_EXCLUSIVE" .It Dv LK_EXCLUSIVE An exclusive lock is held by the current thread. .It Dv LK_EXCLOTHER An exclusive lock is held by someone other than the current thread. .It Dv LK_SHARED A shared lock is held. .It Li 0 The lock is not held by anyone. .El .Sh ERRORS .Fn lockmgr and .Fn lockmgr_rw fail if: .Bl -tag -width Er .It Bq Er EBUSY .Dv LK_FORCEUPGRADE was requested and another thread had already requested a lock upgrade. .It Bq Er EBUSY .Dv LK_NOWAIT was set, and a sleep would have been required, or .Dv LK_TRYUPGRADE operation was not able to upgrade the lock. .It Bq Er ENOLCK .Dv LK_SLEEPFAIL was set and .Fn lockmgr or .Fn lockmgr_rw did sleep. .It Bq Er EINTR .Dv PCATCH was set in the lock priority, and a signal was delivered during a sleep. Note the .Er ERESTART error below. .It Bq Er ERESTART .Dv PCATCH was set in the lock priority, a signal was delivered during a sleep, and the system call is to be restarted. .It Bq Er EWOULDBLOCK a non-zero timeout was given, and the timeout expired. .El .Sh LOCKS If .Dv LK_INTERLOCK is passed in the .Fa flags argument to .Fn lockmgr or .Fn lockmgr_rw , the .Fa ilk must be held prior to calling .Fn lockmgr or .Fn lockmgr_rw , and will be returned unlocked. .Pp Upgrade attempts that fail result in the loss of the lock that is currently held. Also, it is invalid to upgrade an exclusive lock, and a .Xr panic 9 will be the result of trying. .Sh SEE ALSO .Xr condvar 9 , .Xr locking 9 , .Xr mtx_assert 9 , .Xr mutex 9 , .Xr panic 9 , .Xr rwlock 9 , .Xr sleep 9 , .Xr sx 9 , .Xr VOP_PRINT 9 .Sh AUTHORS This manual page was written by .An Chad David Aq Mt davidc@acns.ab.ca .