Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
RWLOCK(9)		 BSD Kernel Developer's	Manual		     RWLOCK(9)

     rwlock, rw_init, rw_init_flags, rw_destroy, rw_rlock, rw_wlock,
     rw_runlock, rw_wunlock, rw_try_rlock, rw_try_upgrade, rw_try_wlock,
     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_try_rlock(struct rwlock	*rw);

     rw_try_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);

     options INVARIANTS

     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 us-
     age 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 mu-
     texes, 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 ex-
     clusive 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, de-
	     scribed by	name name.  The	description is used solely for debug-
	     ging purposes.  This function must	be called before any other op-
	     erations 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	fol-
	     lowing flags:

	     RW_DUPOK	   Witness should not log messages about duplicate
			   locks being acquired.

	     RW_NOPROFILE  Do not profile this lock.

	     RW_NOWITNESS  Instruct witness(4) to ignore this lock.

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

	     RW_RECURSE	   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_try_rlock(struct rwlock	*rw)
	     Try to lock rw as a reader.  This function	will return true if
	     the operation succeeds, otherwise 0 will be returned.

     rw_try_wlock(struct rwlock	*rw)
	     Try to lock rw as a writer.  This function	will return true if
	     the operation succeeds, otherwise 0 will be returned.

     rw_runlock(struct rwlock *rw)
	     This function releases a shared lock previously acquired by

     rw_wunlock(struct rwlock *rw)
	     This function releases an exclusive lock previously acquired by

     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 de-
	     tails 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 com-
	     piled with	options	INVARIANTS and options INVARIANT_SUPPORT, the
	     kernel will panic.	 Currently the following assertions are	sup-

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

	     RA_RLOCKED	  Assert that current thread holds a shared lock of

	     RA_WLOCKED	  Assert that current thread holds an exclusive	lock
			  of rw.

	     RA_UNLOCKED  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.

BSD				August 8, 2008				   BSD


Want to link to this manual page? Use this URL:

home | help