Provided by: freebsd-manpages_12.0-1_all bug


     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 sleep(9).

     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 functionality within the
     kernel, including support for shared and exclusive locks, and recursion.  lockmgr() and
     lockmgr_rw() are also able to upgrade 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.  Downgrading a shared lock
                            is not permitted.  If an exclusive lock has been recursed, the system
                            will panic(9).

            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 upgrade, 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

            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

            LK_INTERLOCK    Unlock the interlock (which should be locked already).

            LK_NODDLKTREAT  Normally, 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.

                            The 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 vp locking
                            in VFS lookup(9), when dvp is already locked.

     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 basis.  The specified values will
     override the default ones, but this can still be used passing, respectively,

     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 otherwise.

     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 specified:

     KA_LOCKED    Assert that the current thread has either a shared or an exclusive lock on the
                  lkp lock pointed to by the first argument.

     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 lkp.

     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 thread.

     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 required, or LK_TRYUPGRADE
                        operation was not able to upgrade the lock.

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

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

     [ERESTART]         PCATCH was set in the lock priority, a signal was delivered during a
                        sleep, and the system call is to be restarted.

     [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 currently 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),


     This manual page was written by Chad David <>.