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

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

     #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(struct lock *lkp);

     lockmgr_recursed(struct lock *lkp);

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

     lockmgr_waiters(struct lock *lkp);

     lockstatus(struct lock *lkp);

     options INVARIANTS

     lockmgr_assert(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_ADAPTIVE	   Enable adaptive spinning for	this lock if the ker-
			   nel is compiled with	the ADAPTIVE_LOCKMGRS option.

	    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, it will be downgraded.

	    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, all references will
			   be downgraded.

	    LK_UPGRADE	   Upgrade a shared lock to an exclusive lock.	If
			   this	call fails, the	shared lock is lost.  During
			   the upgrade,	the shared lock	could be temporarily
			   dropped.  Attempts to upgrade an exclusive lock
			   will	cause a	panic(9).

	    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 freeing 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-

     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 lockmgr_waiters() function returns true if the	lock has waiters, 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-

     [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), mutex(9), rwlock(9), sleep(9), sx(9),
     mtx_assert(9), panic(9), VOP_PRINT(9)

     This manual page was written by Chad David	<>.

BSD				 June 16, 2009				   BSD


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