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RWLOCK(9)	       FreeBSD Kernel Developer's Manual	     RWLOCK(9)

NAME
     rwlock, rw_init, rw_init_flags, rw_destroy, rw_rlock, rw_wlock,
     rw_runlock, rw_wunlock, rw_unlock,	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

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

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

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

     void
     rw_destroy(struct rwlock *rw);

     void
     rw_rlock(struct rwlock *rw);

     void
     rw_wlock(struct rwlock *rw);

     int
     rw_try_rlock(struct rwlock	*rw);

     int
     rw_try_wlock(struct rwlock	*rw);

     void
     rw_runlock(struct rwlock *rw);

     void
     rw_wunlock(struct rwlock *rw);

     void
     rw_unlock(struct rwlock *rw);

     int
     rw_try_upgrade(struct rwlock *rw);

     void
     rw_downgrade(struct rwlock	*rw);

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

     int
     rw_initialized(const struct rwlock	*rw);

     int
     rw_wowned(const struct rwlock *rw);

     options INVARIANTS
     options INVARIANT_SUPPORT

     void
     rw_assert(const struct rwlock *rw,	int what);

     #include <sys/kernel.h>

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

DESCRIPTION
     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 writers.  This limitation
     comes from	the fact that readers are anonymous.  Another important	prop-
     erty is that readers can always 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	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_rlock().

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

     rw_unlock(struct rwlock *rw)
	     This function releases a shared lock previously acquired by
	     rw_rlock()	or 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(const 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(const struct rwlock *rw)
	     This function returns a non-zero value if the current thread owns
	     an	exclusive lock on rw.

     rw_assert(const 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 base assertions are
	     supported:

	     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
			  rw.

	     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.

	     In	addition, one of the following optional	flags may be specified
	     with RA_LOCKED, RA_RLOCKED, or RA_WLOCKED:

	     RA_RECURSED     Assert that the current thread holds a recursive
			     lock of rw.

	     RA_NOTRECURSED  Assert that the current thread does not hold a
			     recursive lock of rw.

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

HISTORY
     These functions appeared in FreeBSD 7.0.

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

BUGS
     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.

FreeBSD	9.2			 June 20, 2013			   FreeBSD 9.2

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY | AUTHORS | BUGS

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