RWLOCK(9) MidnightBSD Kernel Developer’s Manual RWLOCK(9)


rwlock, rw_init, rw_init_flags, rw_destroy, rw_rlock, rw_wlock, rw_runlock, rw_wunlock, rw_try_upgrade, rw_downgrade, rw_sleep, rw_initialized, rw_wowned, rw_assert, RW_SYSINIT — kernel reader/writer lock


#include <sys/param.h>
#include <sys/lock.h>
#include <sys/rwlock.h>


rw_init(struct rwlock *rw, const char *name);


rw_init_flags(struct rwlock *rw, const char *name, int opts);


rw_destroy(struct rwlock *rw);


rw_rlock(struct rwlock *rw);


rw_wlock(struct rwlock *rw);


rw_runlock(struct rwlock *rw);


rw_wunlock(struct rwlock *rw);


rw_try_upgrade(struct rwlock *rw);


rw_downgrade(struct rwlock *rw);


rw_sleep(void *chan, struct rwlock *rw, int priority, const char *wmesg, int timo);


rw_initialized(struct rwlock *rw);


rw_wowned(struct rwlock *rw);



rw_assert(struct rwlock *rw, int what);

#include <sys/kernel.h>

RW_SYSINIT(name, struct rwlock *rw, const char *desc);


Reader/writer locks allow shared access to protected data by multiple threads, or exclusive access by a single thread. The threads with shared access are known as readers since they only read the protected data. A thread with exclusive access is known as a writer since it can modify protected data.

Although reader/writer locks look very similar to sx(9) locks, their usage pattern is different. Reader/writer locks can be treated as mutexes (see mutex(9)) with shared/exclusive semantics. Unlike sx(9), an rwlock can be locked while holding a non-spin mutex, and an rwlock cannot be held while sleeping. The rwlock locks have priority propagation like mutexes, but priority can be propagated only to an exclusive holder. This limitation comes from the fact that shared owners are anonymous. Another important property is that shared holders of rwlock can recurse, and exclusive locks can be made recursive selectively.

Macros and Functions

rw_init(struct rwlock *rw, const char *name)

Initialize structure located at rw as reader/writer lock, described by name name. The description is used solely for debugging purposes. This function must be called before any other operations on the lock.

rw_init_flags(struct rwlock *rw, const char *name, int opts)

Initialize the rw lock just like the rw_init() function, but specifying a set of optional flags to alter the behaviour of rw, through the opts argument. It contains one or more of the following flags:


Witness should not log messages about duplicate locks being acquired.


Do not profile this lock.


Instruct witness(4) to ignore this lock.


Do not log any operations for this lock via ktr(4).


Allow threads to recursively acquire exclusive locks for rw.

rw_rlock(struct rwlock *rw)

Lock rw as a reader. If any thread holds this lock exclusively, the current thread blocks, and its priority is propagated to the exclusive holder. The rw_rlock() function can be called when the thread has already acquired reader access on rw. This is called ‘‘recursing on a lock’’.

rw_wlock(struct rwlock *rw)

Lock rw as a writer. If there are any shared owners of the lock, the current thread blocks. The rw_wlock() function can be called recursively only if rw has been initialized with the RW_RECURSE option enabled.

rw_runlock(struct rwlock *rw)

This function releases a shared lock previously acquired by rw_rlock().

rw_wunlock(struct rwlock *rw)

This function releases an exclusive lock previously acquired by rw_wlock().

rw_try_upgrade(struct rwlock *rw)

Attempt to upgrade a single shared lock to an exclusive lock. The current thread must hold a shared lock of rw. This will only succeed if the current thread holds the only shared lock on rw, and it only holds a single shared lock. If the attempt succeeds rw_try_upgrade() will return a non-zero value, and the current thread will hold an exclusive lock. If the attempt fails rw_try_upgrade() will return zero, and the current thread will still hold a shared lock.

rw_downgrade(struct rwlock *rw)

Convert an exclusive lock into a single shared lock. The current thread must hold an exclusive lock of rw.

rw_sleep(void *chan, struct rwlock *rw, int priority, const char *wmesg, int timo)

Atomically release rw while waiting for an event. For more details on the parameters to this function, see sleep(9).

rw_initialized(struct rwlock *rw)

This function returns non-zero if rw has been initialized, and zero otherwise.

rw_destroy(struct rwlock *rw)

This functions destroys a lock previously initialized with rw_init(). The rw lock must be unlocked.

rw_wowned(struct rwlock *rw)

This function returns a non-zero value if the current thread owns an exclusive lock on rw.

rw_assert(struct rwlock *rw, int what)

This function allows assertions specified in what to be made about rw. If the assertions are not true and the kernel is compiled with options INVARIANTS and options INVARIANT_SUPPORT, the kernel will panic. Currently the following assertions are supported:


Assert that current thread holds either a shared or exclusive lock of rw.


Assert that current thread holds a shared lock of rw.


Assert that current thread holds an exclusive lock of rw.


Assert that current thread holds neither a shared nor exclusive lock of rw.


locking(9), mutex(9), panic(9), sema(9), sx(9)


These functions appeared in FreeBSD 7.0.


The rwlock facility was written by John Baldwin. This manual page was written by Gleb Smirnoff.


If WITNESS is not included in the kernel, then it is impossible to assert that the current thread does or does not hold a read lock. In the non-WITNESS case, the RA_LOCKED and RA_RLOCKED assertions merely check that some thread holds a read lock.

Reader/writer is a bit of an awkward name. An rwlock can also be called a ‘‘Robert Watson’’ lock if desired.

MidnightBSD 0.3 November 25, 2007 MidnightBSD 0.3