rm_rlock man page on FreeBSD

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RMLOCK(9)		 BSD Kernel Developer's Manual		     RMLOCK(9)

NAME
     rmlock, rm_init, rm_init_flags, rm_destroy, rm_rlock, rm_wlock,
     rm_runlock, rm_wunlock, rm_wowned, RM_SYSINIT — kernel reader/writer lock
     optimized for mostly read access patterns

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

     void
     rm_init(struct rmlock *rm, const char *name);

     void
     rm_init_flags(struct rmlock *rm, const char *name, int opts);

     void
     rm_destroy(struct rmlock *rm);

     void
     rm_rlock(struct rmlock *rm, struct rm_priotracker* tracker);

     void
     rm_wlock(struct rmlock *rm);

     void
     rm_runlock(struct rmlock *rm, struct rm_priotracker* tracker);

     void
     rm_wunlock(struct rmlock *rm);

     int
     rm_wowned(struct rmlock *rm);

     #include <sys/kernel.h>

     RM_SYSINIT(name, struct rmlock *rm, const char *desc, int opts);

DESCRIPTION
     Mostly reader 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.

     Read mostly locks are designed to be efficient for locks almost exclu‐
     sively used as reader locks and as such should be used for protecting
     data that rarely changes.	Acquiring an exclusive lock after the lock had
     been locked for shared access is an expensive operation.

     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
     rmlock can be locked while holding a non-spin mutex, and an rmlock cannot
     be held while sleeping.  The rmlock locks have full priority propagation
     like mutexes.  The rm_priotracker structure argument supplied in
     rm_rlock() and rm_runlock() is used to keep track of the read owner(s).
     Another important property is that shared holders of rmlock can recurse
     if the lock has been initialized with the LO_RECURSABLE option, however
     exclusive locks are not allowed to recurse.

   Macros and Functions
     rm_init(struct rmlock *rm, const char *name)
	     Initialize structure located at rm as mostly reader lock,
	     described by name.	 The name description is used solely for
	     debugging purposes.  This function must be called before any
	     other operations on the lock.

     rm_init_flags(struct rmlock *rm, const char *name, int opts)
	     Initialize the rm lock just like the rm_init() function, but
	     specifying a set of optional flags to alter the behaviour of rm,
	     through the opts argument.	 It contains one or more of the fol‐
	     lowing flags:

	     RM_NOWITNESS  Instruct witness(4) to ignore this lock.

	     RM_RECURSE	   Allow threads to recursively acquire exclusive
			   locks for rm.

     rm_rlock(struct rmlock *rm, struct rm_priotracker* tracker)
	     Lock rm as a reader.  Using tracker to track read owners of a
	     lock for priority propagation.  This data structure is only used
	     internally by rmlock and must persist until rm_runlock() has been
	     called.  This data structure can be allocated on the stack since
	     rmlocks cannot be held while sleeping.  If any thread holds this
	     lock exclusively, the current thread blocks, and its priority is
	     propagated to the exclusive holder.  If the lock was initialized
	     with the LO_RECURSABLE option the rm_rlock() function can be
	     called when the thread has already acquired reader access on rm.
	     This is called “recursing on a lock”.

     rm_wlock(struct rmlock *rm)
	     Lock rm as a writer.  If there are any shared owners of the lock,
	     the current thread blocks.	 The rm_wlock() function cannot be
	     called recursively.

     rm_runlock(struct rmlock *rm, struct rm_priotracker* tracker)
	     This function releases a shared lock previously acquired by
	     rm_rlock().  The tracker argument must match the tracker argument
	     used for acquiring the shared lock

     rm_wunlock(struct rmlock *rm)
	     This function releases an exclusive lock previously acquired by
	     rm_wlock().

     rm_destroy(struct rmlock *rm)
	     This functions destroys a lock previously initialized with
	     rm_init().	 The rm lock must be unlocked.

     rm_wowned(struct rmlock *rm)
	     This function returns a non-zero value if the current thread owns
	     an exclusive lock on rm.

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

HISTORY
     These functions appeared in FreeBSD 7.0.

AUTHORS
     The rmlock facility was written by Stephan Uphoff.	 This manual page was
     written by Gleb Smirnoff for rwlock and modified to reflect rmlock by
     Stephan Uphoff.

BUGS
     The rmlock implementation is currently not optimized for single processor
     systems.

     The rmlock implementation uses a single per CPU list shared by all
     rmlocks in the system.  If rmlocks become popular, hashing to multiple
     per CPU queues may be needed to speed up the writer lock process.

     The rmlock can currently not be used as a lock argument for condition
     variable wait functions.

BSD			       November 10, 2007			   BSD
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