RWLOCK(9) FreeBSD Kernel Developer's Manual RWLOCK(9)
NAME
rwlock, rw_init, rw_init_flags, rw_enter, rw_exit, rw_enter_read,
rw_enter_write, rw_exit_read, rw_exit_write, rw_assert_wrlock,
rw_assert_rdlock, rw_assert_anylock, rw_assert_unlocked, rw_status,
RWLOCK_INITIALIZER, rrw_init, rrw_init_flags, rrw_enter, rrw_exit,
rrw_status - interface to read/write locks
SYNOPSIS
#include <sys/rwlock.h>
void
rw_init(struct rwlock *rwl, const char *name);
void
rw_init_flags(struct rwlock *rwl, const char *name, int flags);
int
rw_enter(struct rwlock *rwl, int flags);
void
rw_exit(struct rwlock *rwl);
void
rw_enter_read(struct rwlock *rwl);
void
rw_enter_write(struct rwlock *rwl);
void
rw_exit_read(struct rwlock *rwl);
void
rw_exit_write(struct rwlock *rwl);
void
rw_assert_wrlock(struct rwlock *rwl);
void
rw_assert_rdlock(struct rwlock *rwl);
void
rw_assert_anylock(struct rwlock *rwl);
void
rw_assert_unlocked(struct rwlock *rwl);
int
rw_status(struct rwlock *rwl);
RWLOCK_INITIALIZER(const char *name);
void
rrw_init(struct rrwlock *rrwl, const char *name);
void
rrw_init_flags(struct rrwlock *rrwl, const char *name, int flags);
int
rrw_enter(struct rrwlock *rrwl, int flags);
void
rrw_exit(struct rrwlock *rrwl);
int
rrw_status(struct rrwlock *rrwl);
DESCRIPTION
The rwlock set of functions provides a multiple-reader, single-writer
locking mechanism to ensure mutual exclusion between different threads.
Read locks can be acquired while the write lock is not held, and may
coexist in distinct threads at any time. A write lock, however, can only
be acquired when there are no read locks held, granting exclusive access
to a single thread.
The rw_init() function is used to initiate the lock pointed to by rwl.
The name argument specifies the name of the lock, which is used as the
wait message if the thread needs to sleep.
The rw_init_flags() macro is similar to rw_init(), but it additionally
accepts a bitwise OR of the following flags:
RWL_DUPOK Prevents witness(4) from logging when a thread
acquires more than one lock of this lock type.
RWL_IS_VNODE Make witness(4) ignore lock order issues between
this lock type and any other lock type tagged with
the RWL_IS_VNODE flag.
RWL_NOWITNESS Instructs witness(4) to ignore this lock.
The rw_enter() function acquires a lock. The flags argument specifies
what kind of lock should be obtained and also modifies the operation.
The possible flags are:
RW_READ Acquire a shared lock.
RW_WRITE Acquire an exclusive lock.
RW_DOWNGRADE Safely release an exclusive lock and acquire a
shared lock without letting other exclusive locks
in between.
RW_UPGRADE Upgrade a shared lock into an exclusive one. Must
be combined with RW_NOSLEEP.
RW_INTR When waiting for a lock, allow signals to
interrupt the sleep.
RW_NOSLEEP Do not wait for busy locks, fail with EBUSY
instead.
RW_SLEEPFAIL Wait for busy locks, but do not obtain them, fail
with EAGAIN instead.
RW_DUPOK Prevents witness(4), for just this rw_enter(),
from logging when this thread already has a lock
of this lock type.
The rw_exit() function is used to release a held lock.
The rw_enter_read() function acquires a read lock, sleeping if necessary.
The rw_enter_write() function acquires a write lock, sleeping if
necessary.
The rw_exit_read() function releases a read lock.
The rw_exit_write() function releases a write lock.
The rw_assert_wrlock(), rw_assert_rdlock(), rw_assert_anylock(), and
rw_assert_unlocked() functions check the status rwl, panicking if it is
not write-, read-, any-, or unlocked, respectively.
rw_status returns the current state of the lock.
A lock declaration may be initialised with the RWLOCK_INITIALIZER()
macro. The name argument specifies the name of the lock, which is used
as the wait message if the thread needs to sleep.
The rrwlock functions support recursive write locking by the same
process. They otherwise behave the same as their rwlock counterparts.
CONTEXT
rw_init(), rw_init_flags(), rrw_init() and rrw_init_flags() can be called
during autoconf, from process context, or from interrupt context.
All other functions can be called during autoconf or from process
context.
RETURN VALUES
rw_enter and rrw_enter return 0 on success, or an errno(2) style value on
failure.
rw_status and rrw_status return the state of the lock:
RW_WRITE Lock is write locked by the calling thread.
RW_WRITE_OTHER Lock is write locked by a different thread.
RW_READ Lock is read locked. The current thread may be one
of the threads that has it locked.
0 Lock is not locked.
SEE ALSO
witness(4), mutex(9), rwsleep(9), spl(9)
HISTORY
The rwlock functions first appeared in OpenBSD 3.5.
AUTHORS
The rwlock functions were written by Artur Grabowski <
[email protected]>.
FreeBSD 14.1-RELEASE-p8 January 29, 2025 FreeBSD 14.1-RELEASE-p8