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

     rwlock, rw_init, rw_init_flags, rw_destroy, rw_rlock, rw_wlock,
     rw_runlock, rw_wunlock, rw_try_upgrade, rw_downgrade, rw_sleep,
     rw_initialized, rw_wowned, rw_assert, RW_SYSINIT - kernel reader/writer

     #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_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

             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

             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_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

             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 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 compiled with
                               options INVARIANTS and options
                               INVARIANT_SUPPORT, the kernel will panic.
                               Currently the following assertions are

                               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.

     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.

FreeBSD 11.0-PRERELEASE        November 25, 2007       FreeBSD 11.0-PRERELEASE


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