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Man Pages


Manual Reference Pages  -  RW_DOWNGRADE (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

CONTENTS

Synopsis
Description
     Macros and Functions
See Also
History
Authors
Bugs

SYNOPSIS


.In sys/param.h
.In sys/lock.h
.In 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


.Cd options INVARIANTS
.Cd options INVARIANT_SUPPORT void rw_assert const struct rwlock *rw int what
.In 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
.Cd options INVARIANTS and
.Cd 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
.Fx 7.0 .

AUTHORS


.An -nosplit The rwlock facility was written by
.An John Baldwin . This manual page was written by
.An 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.

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