LOCK(9)

NAME
lockinit, lockmgr, lockmgr_try, lockstatus, lockmgr_printinfo, lockowned - lockmgr family of functions

SYNOPSIS
#include <sys/types.h>
#include <sys/lock.h>
void
lockinit(struct lock *lkp, const char *wmesg, int timo, int flags);
void
lockuninit(struct lock *lkp);
int
lockmgr(struct lock *lkp, u_int flags);
boolean_t
lockmgr_try(struct lock *lkp, u_int flags);
int
lockstatus(struct lock *lkp, struct thread *td);
void
lockmgr_printinfo(struct lock *lkp);
int
lockowned(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.

wmesg
    The lock message. This is used for both debugging output and tsleep(9).

timo
    The timeout value passed to tsleep(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_TIMELOCK
        Use timo during a sleep; otherwise, 0 is used.

The lockuninit() function destroys a lock that was previously initialized using lockinit().

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 for any reason, the shared lock is lost. Attempts to upgrade an exclusive lock not already owned by the caller will cause a panic(9), but otherwise will always succeed. NOTE! When this operation succeeds, it guarantees that no other exclusive lock was able to acquire the lock ahead of you, but as indicated above, if it fails your current shared lock is lost.

    LK_UPGRADE
        Upgrade a shared lock to an exclusive lock. If this call fails for any reason, the shared lock is lost. Attempts to upgrade an exclusive lock not already owned by the caller will cause a panic(9), but otherwise will always succeed. WARNING! This operation can block with the current lock temporarily released, and other exclusive or shared lock holders can inject before the lock is acquired on your behalf.

    LK_RELEASE
        Release the lock. Releasing a lock that is not held can cause a panic(9).

    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.

The lockmgr_try() function is similar to lockmgr() but it will not sleep and returns TRUE if the lock was successfully obtained and FALSE if it was not.

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 lockowned() function is used to determine whether the calling thread owns a lock.

RETURN VALUES
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.

The lockowned() function returns a non-zero value if the caller owns the lock shared or exclusive.

FILES
/sys/kern/kern_lock.c
    the lock manager implementation
/sys/sys/lock.h
    data structures and function prototypes for the lock manager

ERRORS
lockmgr() fails if:

[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
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
locking(9), panic(9), tsleep(9), VOP_PRINT(9)

HISTORY
The lock manager appeared in DragonFly 1.0.

The lock manager API first appeared in 4.4BSD lite2.

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