LOCK(9) MidnightBSD Kernel Developer’s Manual LOCK(9)

NAME

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

SYNOPSIS

#include <sys/types.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

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 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_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 <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 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

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〉.

MidnightBSD 0.3 June 20, 2006 MidnightBSD 0.3