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

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

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

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

     void
     lockdestroy(struct lock *lkp);

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

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

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

     void
     lockmgr_disown(struct lock *lkp);

     void
     lockmgr_printinfo(const struct lock *lkp);

     int
     lockmgr_recursed(const struct lock *lkp);

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

     int
     lockmgr_waiters(const struct lock *lkp);

     int
     lockstatus(const struct lock *lkp);

     options INVARIANTS
     options INVARIANT_SUPPORT
     void
     lockmgr_assert(const struct lock *lkp, int what);

DESCRIPTION
     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_ADAPTIVE        Enable adaptive spinning for this lock if
                                   the kernel 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
     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 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 <sys/lockmgr.h>.)

                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 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
                                    unneccessary 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, LK_WMESG_DEFAULT, LK_PRIO_DEFAULT
     and LK_TIMO_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
     otherwise.

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

RETURN VALUES
     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.

ERRORS
     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.

LOCKS
     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.

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

AUTHORS
     This manual page was written by Chad David <davidc@acns.ab.ca>.

FreeBSD 11.0-PRERELEASE        November 2, 2014        FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | LOCKS | SEE ALSO | AUTHORS

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