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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
     exclusively 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
             following 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.