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(struct rwlock *rw);

     int
     rw_wowned(struct rwlock *rw);

     options INVARIANTS
     options INVARIANT_SUPPORT

     void
     rw_assert(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 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_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(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 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.