RWLOCK(9) | Kernel Developer's Manual | RWLOCK(9) |
rw
, rw_init
,
rw_destroy
, rw_enter
,
rw_exit
, rw_tryenter
,
rw_tryupgrade
, rw_downgrade
,
rw_read_held
, rw_write_held
,
rw_lock_held
—
#include <sys/rwlock.h>
void
rw_init
(krwlock_t
*rw);
void
rw_destroy
(krwlock_t
*rw);
void
rw_enter
(krwlock_t
*rw, const krw_t
op);
void
rw_exit
(krwlock_t
*rw);
int
rw_tryenter
(krwlock_t
*rw, const krw_t
op);
int
rw_tryupgrade
(krwlock_t
*rw);
void
rw_downgrade
(krwlock_t
*rw);
int
rw_read_held
(krwlock_t
*rw);
int
rw_write_held
(krwlock_t
*rw);
int
rw_lock_held
(krwlock_t
*rw);
options DIAGNOSTIC
options LOCKDEBUG
In addition to the capabilities provided by mutexes, RW locks distinguish between read (shared) and write (exclusive) access.
RW locks are in one of three distinct states at any given time:
Unlocked
Read
locked
Write
locked
The krwlock_t type provides storage for the RW lock object. This should be treated as an opaque object and not examined directly by consumers.
Note that these interfaces must not be used from a hardware interrupt handler.
options DIAGNOSTIC
Kernels compiled with the DIAGNOSTIC
option perform basic sanity checks on RW lock operations.
options LOCKDEBUG
Kernels compiled with the LOCKDEBUG
option perform potentially CPU intensive sanity checks on RW lock
operations.
rw_init
(rw)Initialize a lock for use. No other operations can be performed on the lock until it has been initialized.
rw_destroy
(rw)Release resources used by a lock. The lock may not be used after it has been destroyed.
rw_enter
(rw,
op)If RW_READER
is specified as the
argument to op, acquire a read lock. If the lock
is write held, the caller will block and not return until the hold is
acquired. Callers must not recursively acquire read locks.
If RW_WRITER
is specified, acquire a
write lock. If the lock is already held, the caller will block and not
return until the hold is acquired.
RW locks and other types of locks must always be acquired in a consistent order with respect to each other. Otherwise, the potential for system deadlock exists.
rw_exit
(rw)Release a lock. The lock must have been previously acquired by the caller.
rw_tryenter
(rw,
op)Try to acquire a lock, but do not block if the lock is already held. If the lock is acquired successfully, return non-zero. Otherwise, return zero.
Valid arguments to op are
RW_READER
or
RW_WRITER
.
rw_tryupgrade
(rw)Try to upgrade a lock from one read hold to a write hold. If the lock is upgraded successfully, returns non-zero. Otherwise, returns zero.
rw_downgrade
(rw)Downgrade a lock from a write hold to a read hold.
rw_write_held
(rw)Return non-zero if write lock is held by current lwp. Otherwise, return zero.
rw_read_held
(rw)rw_lock_held
(rw)Returns non-zero if either read or write lock is held by any lwp. Otherwise, return zero.
Functions rw_write_held
(),
rw_read_held
(), and
rw_lock_held
() must never be used to make
locking decisions at run time: they are provided only for diagnostic
purposes. They are also not atomic, hence they should only be used to
assert that lock is held. The only exception is
rw_write_held
(), which can be also used safely
to assert that write lock is NOT currently held by current lwp.
Generally speaking, it is better to organise code paths and/or data flows such that fewer and weaker synchronization points are required to ensure correct operation.
The header file sys/sys/rwlock.h describes the public interface, and interfaces that machine-dependent code must provide to support RW locks.
Jim Mauro and Richard McDougall, Solaris Internals: Core Kernel Architecture, Prentice Hall, 2001, ISBN 0-13-022496-0.
December 10, 2018 | NetBSD 9.2 |