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Man Pages


Manual Reference Pages  - LOCK (9)

NAME

lockinit, lockdestroy, lockcount, lockmgr, lockstatus, lockmgr_printinfo - lockmgr family of functions

CONTENTS

Synopsis
Description
Return Values
Errors
Locks
See Also
Authors

SYNOPSIS


.In sys/types.h
.In sys/lockmgr.h void lockinit struct lock *lkp int prio const char *wmesg int timo int flags void lockdestroy struct lock *lkp int lockcount struct lock *lkp int lockmgr struct lock *lkp u_int flags struct mtx *interlkp struct thread *td int lockstatus struct lock *lkp struct thread *td void lockmgr_printinfo struct lock *lkp

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 msleep(9).
wmesg The lock message. This is used for both debugging output and msleep(9).
timo The timeout value passed to msleep(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 panic(9).
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
.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_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).
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
.Vt 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.

RETURN VALUES

The lockcount function returns an integer greater than or equal to zero.

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 td.
LK_SHARED A shared lock is held.
0 The lock is not held by anyone.

ERRORS

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 required.
[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 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, the interlkp must be held prior to calling lockmgr, 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

msleep(9), mtx_assert(9), panic(9), VOP_PRINT(9)

AUTHORS

This manual page was written by
.An Chad David Aq davidc@acns.ab.ca .
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June 20, 2006 LOCK (9)

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