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

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(const struct rwlock *rw);

     int
     rw_wowned(const struct rwlock *rw);

     options INVARIANTS
     options INVARIANT_SUPPORT
     void
     rw_assert(const 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 writers.  This limitation
     comes from the fact that readers are anonymous.  Another important
     property is that readers can always 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(const 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(const struct rwlock *rw)
             This function returns a non-zero value if the current thread owns
             an exclusive lock on rw.

     rw_assert(const 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.

FreeBSD 11.0-PRERELEASE          June 20, 2013         FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY | AUTHORS | BUGS

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