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author | jhb <jhb@FreeBSD.org> | 2006-04-19 20:43:35 +0000 |
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committer | jhb <jhb@FreeBSD.org> | 2006-04-19 20:43:35 +0000 |
commit | aa021f014585072f724f2218362dcedfd3038b6c (patch) | |
tree | d09e04aad2ebfc603f5d1cbeff388bff4eb75268 /share/man/man9/rwlock.9 | |
parent | a74c12c708597c26029dca15c1c4b95221842e00 (diff) | |
download | FreeBSD-src-aa021f014585072f724f2218362dcedfd3038b6c.zip FreeBSD-src-aa021f014585072f724f2218362dcedfd3038b6c.tar.gz |
Various nits and fixups so that the text reads smoother in several places.
Also, more closely match the prototypes in sys/rwlock.h and add a BUGS
section.
Diffstat (limited to 'share/man/man9/rwlock.9')
-rw-r--r-- | share/man/man9/rwlock.9 | 149 |
1 files changed, 80 insertions, 69 deletions
diff --git a/share/man/man9/rwlock.9 b/share/man/man9/rwlock.9 index 7dae9d8..5411adf 100644 --- a/share/man/man9/rwlock.9 +++ b/share/man/man9/rwlock.9 @@ -24,11 +24,11 @@ .\" .\" $FreeBSD$ .\" -.Dd January 30, 2006 +.Dd April 19, 2006 .Dt RWLOCK 9 .Os .Sh NAME -.Nm rwlock +.Nm rwlock , .Nm rw_init , .Nm rw_rlock , .Nm rw_wlock , @@ -38,118 +38,117 @@ .Nm rw_initialized , .Nm rw_destroy , .Nm RW_SYSINIT , -.Nd kernel synchronization primitives +.Nd kernel reader/writer lock .Sh SYNOPSIS .In sys/param.h .In sys/lock.h .In sys/rwlock.h .Ft void -.Fn rw_init "struct rwlock *rwlock" "const char *name" +.Fn rw_init "struct rwlock *rw" "const char *name" .Ft void -.Fn rw_rlock "struct rwlock *rwlock" +.Fn rw_rlock "struct rwlock *rw" .Ft void -.Fn rw_wlock "struct rwlock *rwlock" +.Fn rw_wlock "struct rwlock *rw" .Ft void -.Fn rw_runlock "struct rwlock *rwlock" +.Fn rw_runlock "struct rwlock *rw" .Ft void -.Fn rw_wunlock "struct rwlock *rwlock" +.Fn rw_wunlock "struct rwlock *rw" .Ft int -.Fn rw_initialized "struct rwlock *rwlock" +.Fn rw_initialized "struct rwlock *rw" .Ft void -.Fn rw_destroy "struct rwlock *rwlock" +.Fn rw_destroy "struct rwlock *rw" .Pp .Cd "options INVARIANTS" .Cd "options INVARIANT_SUPPORT" .Ft void -.Fn rw_assert "struct rwlock *rwlock" "int what" +.Fn rw_assert "struct rwlock *rw" "int what" .In sys/kernel.h -.Fn RW_SYSINIT "name" "struct rwlock *rwlock" "const char *description" +.Fn RW_SYSINIT "name" "struct rwlock *rw" "const char *desc" .Sh DESCRIPTION -Read/write locks are a method of thread synchronization, which -allows several threads have shared access to protected data, or -one thread have exclusive access. -The threads that share access are known as +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 .Em readers -since they should only read the protected data, while thread -with exclusive access is known as a +since they only read the protected data. +A thread with exclusive access is known as a .Em writer since it can modify protected data. .Pp -Although the description of read/write locks looks very similar -to description of +Although reader/writeer locks look very similar to .Xr sx 9 locks, their usage pattern is different. -The read/write locks can be treated as mutexes (see +Reader/writer locks can be treated as mutexes (see .Xr mutex 9 ) with shared/exclusive semantics. Unlike .Xr sx 9 , an .Nm -can be locked while holding a non-spin mutex; +can be locked while holding a non-spin mutex, and an .Nm cannot be held while sleeping. The .Nm locks have priority propagation like mutexes, but priority -can be propagated only to exclusive holder. +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 holder of +Another important property is that shared holders of .Nm -can recurse on it. +can recurse, +but exclusive locks are not allowed to recurse. .Ss Macros and Functions .Bl -tag -width indent -.It Fn rw_init "struct rwlock *rwlock" "const char *name" +.It Fn rw_init "struct rwlock *rw" "const char *name" Initialize structure located at -.Fa rwlock -as read/write lock, described by name +.Fa rw +as reader/writer lock, described by name .Fa name . The description is used solely for debugging purposes. -This function must be used before any other manipulations -with the lock. -.It Fn rw_rlock "struct rwlock *rwlock" -Lock the -.Fa rwlock -as reader. +This function must be called before any other operations +on the lock. +.It Fn rw_rlock "struct rwlock *rw" +Lock +.Fa rw +as a reader. If any thread holds this lock exclusively, the current thread blocks, -and its priority is propagated to exclusive holder. +and its priority is propagated to the exclusive holder. The .Fn rw_rlock function can be called when the thread has already acquired reader access on -.Fa rwlock . +.Fa rw . This is called .Dq "recursing on a lock" . -.It Fn rw_wlock "struct rwlock *rwlock" -Lock the -.Fa rwlock -as writer. +.It Fn rw_wlock "struct rwlock *rw" +Lock +.Fa rw +as a writer. If there are any shared owners of the lock, the current thread blocks. The .Fn rw_wlock function cannot be called recursively. -.It Fn rw_runlock "struct rwlock *rwlock" -This function releases shared lock, previously acquired by +.It Fn rw_runlock "struct rwlock *rw" +This function releases a shared lock previously acquired by .Fn rw_rlock . -.It Fn rw_wunlock "struct rwlock *rwlock" -This function releases exclusive lock, previously acquired by +.It Fn rw_wunlock "struct rwlock *rw" +This function releases an exclusive lock previously acquired by .Fn rw_wlock . -.It Fn rw_initialized "struct rwlock *rwlock" -This function returns non-zero if the -.Fa rwlock +.It Fn rw_initialized "struct rwlock *rw" +This function returns non-zero if +.Fa rw has been initialized, and zero otherwise. -.It Fn rw_destroy "struct rwlock *rwlock" +.It Fn rw_destroy "struct rwlock *rw" This functions destroys a lock previously initialized with .Fn rw_init . The -.Fa rwlock -must be unlocked. -.It Fn rw_assert "struct rwlock *rwlock" "int what" +.Fa rw +lock must be unlocked. +.It Fn rw_assert "struct rwlock *rw" "int what" This function allows assertions specified in .Fa what to be made about -.Fa rwlock . +.Fa rw . If the assertions are not true and the kernel is compiled with .Cd "options INVARIANTS" @@ -159,29 +158,21 @@ the kernel will panic. Currently the following assertions are supported: .Bl -tag -width ".Dv RA_UNLOCKED" .It Dv RA_LOCKED -Assert that current thread is either shared or exclusive owner -of the -.Nm -pointed to by the first argument. +Assert that current thread holds either a shared or exclusive lock +of +.Fa rw . .It Dv RA_RLOCKED -Assert that current thread is shared owner of the -.Nm -pointed -to by the first argument. +Assert that current thread holds a shared lock of +.Fa rw . .It Dv RA_WLOCKED -Assert that current thread is exclusive owner of the -.Nm -pointed -to by the first argument. +Assert that current thread holds an exclusive lock of +.Fa rw . .It Dv RA_UNLOCKED -Assert that current thread is neither shared nor exclusive owner -of the -.Nm -pointed to by the first argument. +Assert that current thread holds neither a shared nor exclusive lock of +.Fa rw . .El .El .Sh SEE ALSO -.Xr condvar 9 , .Xr mutex 9 , .Xr panic 9 , .Xr sema 9 , @@ -198,3 +189,23 @@ facility was written by .An "John Baldwin" . This manual page was written by .An "Gleb Smirnoff" . +.Sh BUGS +If +.Dv WITNESS +is not included in the kernel, +then it is impossible to assert that the current thread does or does not +hold a shared lock. +In the +.No non - Ns Dv WITNESS +case, the +.Dv RA_LOCKED +and +.Dv RA_RLOCKED +assertions merely check that some thread holds a shared lock. +.Pp +Reader/writer is a bit of an awkward name. +An +.Nm +can also be called a +.Dq Robert Watson +lock if desired. |