LOCK(9)

NAME
lock, simple_lock_init, simple_lock, simple_lock_try, simple_unlock, lockinit, lockmgr, lockstatus, lockmgr_printinfo, - kernel lock functions

SYNOPSIS
#include <sys/lock.h>
void
simple_lock_init(struct simplelock *slock);
void
simple_lock(struct simplelock *slock);
int
simple_lock_try(struct simplelock *slock);
void
simple_unlock(struct simplelock *slock);
void
lockinit(struct lock *lock, int prio, const char *wmesg, int timo, int flags);
int
lockmgr(struct lock *lock, u_int flags, struct simplelock *slock);
int
lockstatus(struct lock *lock);
void
lockmgr_printinfo(struct lock *lock);

DESCRIPTION
The lock functions provide synchronisation in the kernel by preventing multiple processes from simultaneously executing critical sections of code accessing shared data. A number of different locks are available:

struct simplelock
    Provides a simple spinning mutex. A processor will busy-wait while trying to acquire a simplelock. The simplelock operations are implemented with machine-dependent locking primitives.

    Simplelocks are usually used only by the high-level lock manager and to protect short, critical sections of code. Simplelocks are the only locks that can be used inside an interrupt handler. For a simplelock to be used in an interrupt handler, care must be taken to disable the interrupt, acquire the lock, do any processing, release the simplelock and re-enable the interrupt. This procedure is necessary to avoid deadlock between the interrupt handler and other processes executing on the same processor.

struct lock
    Provides a high-level lock supporting sleeping/spinning until the lock can be acquired. The lock manager supplies both exclusive-access and shared-access locks, with recursive exclusive-access locks within a single process. It also allows upgrading a shared-access lock to an exclusive-access lock, as well as downgrading an exclusive-access lock to a shared-access lock.

If the kernel option LOCKDEBUG is enabled, additional facilities are provided to record additional lock information. These facilities are provided to assist in determining deadlock occurrences.

FUNCTIONS
The functions which operate on simplelocks are:

simple_lock_init(slock)
    The simplelock slock is initialised to the unlocked state.

simple_lock(slock)
    The simplelock slock is locked. If the simplelock is held then execution will spin until the simplelock is acquired. Care must be taken that the calling process does not already hold the simplelock. In this case, the simplelock can never be acquired. If kernel option LOCKDEBUG is enabled, a "locking against myself" panic will occur.

simple_lock_try(slock)
    Try to acquire the simplelock slock without spinning. If the simplelock is held by another process then the return value is 0. If the simplelock was acquired successfully then the return value is 1.

simple_unlock(slock)
    The simplelock slock is unlocked. The simplelock must be locked and the calling process must be the one that last acquired the simplelock. If the calling process does not hold the simplelock, the simplelock will be released but the kernel behaviour is undefined.

The functions which operate on locks are:

lockinit(lock, prio, wmesg, timo, flags)
    The lock lock is initialised according to the parameters provided. Arguments are as follows:

    lock
        The lock.
    prio
        The process priority when it is woken up after sleeping on the lock.
    wmesg
        A sleep message used when a process goes to sleep waiting for the lock, so that the exact reason it is sleeping can easily be identified.
    timo
        The maximum sleep time. Used by tsleep(9).
    flags
        Flags to specify the lock behaviour permanently over the lifetime of the lock. Valid lock flags are:

        LK_NOWAIT
            Processes should not sleep when attempting to acquire the lock.
        LK_CANRECURSE
            Processes can acquire the lock recursively.

lockmgr(lock, flags, slock)
    Set, change or release a lock according to the parameters provided. Arguments are as follows:

    lock
        The lock.
    flags
        Flags to specify the lock request type. In addition to the flags specified above, the following flags are valid:

        LK_SHARED
            Get one of many possible shared-access locks. If a process holding an exclusive-access lock requests a shared-access lock, the exclusive-access lock is downgraded to a shared-access lock.

        LK_EXCLUSIVE
            Stop further shared-access locks, when they are cleared, grant a pending upgrade if it exists, then grant an exclusive-access lock. Only one exclusive-access lock may exist at a time, except that a process holding an exclusive-access lock may get additional exclusive-access locks if it explicitly sets the LK_CANRECURSE flag in the lock request, or if the LK_CANRECURSE flag was set when the lock was initialised.

        LK_RELEASE
            Release one instance of a lock.

        LK_DRAIN
            Wait for all activity on the lock to end, then mark it decommissioned. This feature is used before freeing a lock that is part of a piece of memory that is about to be freed.

        LK_RECURSEFAIL
            Attempt at recursive lock fails.

    slock
        This argument exists for legacy reasons, it is now ignored.

lockstatus(lock)
    Determine the status of lock lock. Returns LK_EXCLUSIVE or LK_SHARED for exclusive-access and shared-access locks respectively.

lockmgr_printinfo(lock)
    Print out information about state of lock lock.

RETURN VALUES
Successfully acquired locks return 0. A failed lock attempt always returns a non-zero error value. No lock is held after an error return. Locks will always succeed unless one of the following is true:

[EBUSY]
    LK_NOWAIT is set and a sleep would be required.

[EINTR]
    PCATCH is set in lock priority and a signal arrives to interrupt a system call.

[ERESTART]
    PCATCH is set in lock priority and a signal arrives so that the system call is restarted.

[EWOULDBLOCK]
    Non-null lock timeout and timeout expires.

CODE REFERENCES
This section describes places within the OpenBSD source tree where actual code implementing or utilising the locking framework can be found. All pathnames are relative to /usr/src.

The locking framework itself is implemented within the file sys/kern/kern_lock.c. Data structures and function prototypes for the framework are located in sys/sys/lock.h.

SEE ALSO
mutex(9), pmap(9), rwlock(9), spl(9), tsleep(9), uvm(9)

HISTORY
The kernel locking API first appeared in 4.4BSD -lite2. It was progressively deprecated in favor of rwlock(9).