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     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
     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:

             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_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

             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:

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

                           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


     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.