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LOCK(9)		       FreeBSD Kernel Developer's Manual	       LOCK(9)

     lockinit, lockdestroy, lockmgr, lockmgr_args, lockmgr_args_rw,
     lockmgr_disown, lockmgr_printinfo,	lockmgr_recursed, lockmgr_rw,
     lockstatus, lockmgr_assert	-- lockmgr family of functions

     #include <sys/types.h>
     #include <sys/lock.h>
     #include <sys/lockmgr.h>

     lockinit(struct lock *lkp,	int prio, const	char *wmesg, int timo,
	 int flags);

     lockdestroy(struct	lock *lkp);

     lockmgr(struct lock *lkp, u_int flags, struct mtx *ilk);

     lockmgr_args(struct lock *lkp, u_int flags, struct	mtx *ilk,
	 const char *wmesg, int	prio, int timo);

     lockmgr_args_rw(struct lock *lkp, u_int flags, struct rwlock *ilk,
	 const char *wmesg, int	prio, int timo);

     lockmgr_disown(struct lock	*lkp);

     lockmgr_printinfo(const struct lock *lkp);

     lockmgr_recursed(const struct lock	*lkp);

     lockmgr_rw(struct lock *lkp, u_int	flags, struct rwlock *ilk);

     lockstatus(const struct lock *lkp);

     options INVARIANTS

     lockmgr_assert(const struct lock *lkp, int	what);

     The lockinit() function is	used to	initialize a lock.  It must be called
     before any	operation can be performed on a	lock.  Its arguments are:

     lkp    A pointer to the lock to initialize.

     prio   The	priority passed	to sleep(9).

     wmesg  The	lock message.  This is used for	both debugging output and

     timo   The	timeout	value passed to	sleep(9).

     flags  The	flags the lock is to be	initialized with:

	    LK_CANRECURSE  Allow recursive exclusive locks.

	    LK_NOPROFILE   Disable lock	profiling for this lock.

	    LK_NOSHARE	   Allow exclusive locks only.

	    LK_NOWITNESS   Instruct witness(4) to ignore this lock.

	    LK_NODUP	   witness(4) should log messages about	duplicate
			   locks being acquired.

	    LK_QUIET	   Disable ktr(4) logging for this lock.

	    LK_TIMELOCK	   Use timo during a sleep; otherwise, 0 is used.

     The 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.

     The lockmgr() and lockmgr_rw() functions handle general locking function-
     ality within the kernel, including	support	for shared and exclusive
     locks, and	recursion.  lockmgr() and lockmgr_rw() are also	able to	up-
     grade and downgrade locks.

     Their arguments are:

     lkp    A pointer to the lock to manipulate.

     flags  Flags indicating what action is to be taken.

	    LK_SHARED	    Acquire a shared lock.  If an exclusive lock is
			    currently held, EDEADLK will be returned.

	    LK_EXCLUSIVE    Acquire an exclusive lock.	If an exclusive	lock
			    is already held, and LK_CANRECURSE is not set, the
			    system will	panic(9).

	    LK_DOWNGRADE    Downgrade exclusive	lock to	a shared lock.	Down-
			    grading a shared lock is not permitted.  If	an ex-
			    clusive lock has been recursed, the	system will

	    LK_UPGRADE	    Upgrade a shared lock to an	exclusive lock.	 If
			    this call fails, the shared	lock is	lost, even if
			    the	LK_NOWAIT flag is specified.  During the up-
			    grade, the shared lock could be temporarily
			    dropped.  Attempts to upgrade an exclusive lock
			    will cause a panic(9).

	    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.

	    LK_RELEASE	    Release the	lock.  Releasing a lock	that is	not
			    held can cause a panic(9).

	    LK_DRAIN	    Wait for all activity on the lock to end, then
			    mark it decommissioned.  This is used before free-
			    ing	a lock that is part of a piece of memory that
			    is about to	be freed.  (As documented in

	    LK_SLEEPFAIL    Fail if operation has slept.

	    LK_NOWAIT	    Do not allow the call to sleep.  This can be used
			    to test the	lock.

	    LK_NOWITNESS    Skip the witness(4)	checks for this	instance.

	    LK_CANRECURSE   Allow recursion on an exclusive lock.  For every
			    lock there must be a release.

	    LK_INTERLOCK    Unlock the interlock (which	should be locked al-

	    LK_NODDLKTREAT  Normally, lockmgr()	postpones serving further
			    shared requests for	shared-locked lock if there is
			    exclusive waiter, to avoid exclusive lock starva-
			    tion.  But,	if the thread requesting the shared
			    lock already owns a	shared lockmgr lock, the re-
			    quest is granted even in presence of the parallel
			    exclusive lock request, which is done to avoid
			    deadlocks with recursive shared acquisition.

			    The	LK_NODDLKTREAT flag can	only be	used by	code
			    which requests shared non-recursive	lock.  The
			    flag allows	exclusive requests to preempt the cur-
			    rent 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
			    vp locking in VFS lookup(9), when dvp is already

     ilk    An interlock mutex for controlling group access to the lock.  If
	    LK_INTERLOCK is specified, lockmgr() and lockmgr_rw() assume ilk
	    is currently owned and not recursed, and will return it unlocked.
	    See	mtx_assert(9).

     The lockmgr_args()	and lockmgr_args_rw() function work like lockmgr() and
     lockmgr_rw() but accepting	a wmesg, timo and prio on a per-instance ba-
     sis.  The specified values	will override the default ones,	but this can
     still be used passing, respectively, LK_WMESG_DEFAULT, LK_PRIO_DEFAULT

     The lockmgr_disown() function switches the	owner from the current thread
     to	be LK_KERNPROC,	if the lock is already held.

     The lockmgr_printinfo() function prints debugging information about the
     lock.  It is used primarily by VOP_PRINT(9) functions.

     The lockmgr_recursed() function returns true if the lock is recursed, 0

     The lockstatus() function returns the status of the lock in relation to
     the current thread.

     When compiled with	options	INVARIANTS and options INVARIANT_SUPPORT, the
     lockmgr_assert() function tests lkp for the assertions specified in what,
     and panics	if they	are not	met.  One of the following assertions must be

     KA_LOCKED	  Assert that the current thread has either a shared or	an ex-
		  clusive lock on the lkp lock pointed to by the first argu-

     KA_SLOCKED	  Assert that the current thread has a shared lock on the lkp
		  lock pointed to by the first argument.

     KA_XLOCKED	  Assert that the current thread has an	exclusive lock on the
		  lkp lock pointed to by the first argument.

     KA_UNLOCKED  Assert that the current thread has no	lock on	the lkp	lock
		  pointed to by	the first argument.

     In	addition, one of the following optional	assertions can be used with
     either an KA_LOCKED, KA_SLOCKED, or KA_XLOCKED assertion:

     KA_RECURSED     Assert that the current thread has	a recursed lock	on

     KA_NOTRECURSED  Assert that the current thread does not have a recursed
		     lock on lkp.

     The lockmgr() and lockmgr_rw() functions return 0 on success and non-zero
     on	failure.

     The lockstatus() function returns:

     LK_EXCLUSIVE  An exclusive	lock is	held by	the current thread.

     LK_EXCLOTHER  An exclusive	lock is	held by	someone	other than the current

     LK_SHARED	   A shared lock is held.

     0		   The lock is not held	by anyone.

     lockmgr() and lockmgr_rw()	fail if:

     [EBUSY]		LK_FORCEUPGRADE	was requested and another thread had
			already	requested a lock upgrade.

     [EBUSY]		LK_NOWAIT was set, and a sleep would have been re-
			quired,	or LK_TRYUPGRADE operation was not able	to up-
			grade the lock.

     [ENOLCK]		LK_SLEEPFAIL was set and lockmgr() or lockmgr_rw() did

     [EINTR]		PCATCH was set in the lock priority, and a signal was
			delivered during a sleep.  Note	the ERESTART error be-

     [ERESTART]		PCATCH was set in the lock priority, a signal was de-
			livered	during a sleep,	and the	system call is to be

     [EWOULDBLOCK]	a non-zero timeout was given, and the timeout expired.

     If	LK_INTERLOCK is	passed in the flags argument to	lockmgr() or
     lockmgr_rw(), the ilk must	be held	prior to calling lockmgr() or
     lockmgr_rw(), and will be returned	unlocked.

     Upgrade attempts that fail	result in the loss of the lock that is cur-
     rently held.  Also, it is invalid to upgrade an exclusive lock, and a
     panic(9) will be the result of trying.

     condvar(9), locking(9), mtx_assert(9), mutex(9), panic(9),	rwlock(9),
     sleep(9), sx(9), VOP_PRINT(9)

     This manual page was written by Chad David	<>.

FreeBSD	13.0		       November	17, 2017		  FreeBSD 13.0


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