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     lockinit, lockdestroy, lockcount, lockmgr, lockstatus, lockmgr_printinfo
     - 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);

     lockcount(struct lock *lkp);

     lockmgr(struct lock *lkp, u_int flags, struct mtx *interlkp,
             struct thread *td);

     lockstatus(struct lock *lkp, struct thread *td);

     lockmgr_printinfo(struct lock *lkp);


     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_NOWAIT      Do not sleep while acquiring the lock.

            LK_SLEEPFAIL   Fail after a sleep.

            LK_CANRECURSE  Allow recursive exclusive locks.

            LK_NOSHARE     Allow exclusive locks only.

            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 lockcount() function returns a count of the number of exclusive locks
     and shared locks held against the lock lkp.

     The lockmgr() function handles general locking functionality within the
     kernel, including support for shared and exclusive locks, and recursion.
     lockmgr() is also able to upgrade and downgrade locks.

     Its 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.
                               Downgrading a shared lock is not permitted.  If
                               an exclusive lock has been recursed, all
                               references will be downgraded.

               LK_EXCLUPGRADE  Upgrade a shared lock to an exclusive lock.
                               Fails with EBUSY if there is someone ahead of
                               you in line waiting for an upgrade.  If this
                               call fails, the shared lock is lost.  Attempts
                               to upgrade an exclusive lock will cause a

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

               LK_INTERLOCK    Unlock the interlock (which should be locked

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

     td        A thread responsible for this call.  NULL becomes LK_KERNPROC.

     The lockstatus() function returns the status of the lock in relation to
     the thread passed to it.  Note that if td is NULL and an exclusive lock
     is held, LK_EXCLUSIVE will be returned.

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


     The lockcount() function returns an integer greater than or equal to

     The lockmgr() function returns 0 on success and non-zero on failure.

     The lockstatus() function returns:

     LK_EXCLUSIVE  An exclusive lock is held by the thread td.

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

     LK_SHARED     A shared lock is held.

     0             The lock is not held by anyone.


     lockmgr() fails 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

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

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

     [ERESTART]         PCATCH was set in the lock priority, a signal was
                        delivered 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(), the
     interlkp must be held prior to calling lockmgr(), and will be returned

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


     This manual page was written by Chad David 〈〉.