LOCK(9) OpenBSD Kernel Manual 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 ALSOmutex(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).
OpenBSD 4.9 March 25, 2009 OpenBSD 4.9