338 lines
9.3 KiB
Groff
338 lines
9.3 KiB
Groff
.\" Copyright (c) 2006 Gleb Smirnoff <glebius@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd November 11, 2017
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.Dt RWLOCK 9
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.Os
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.Sh NAME
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.Nm rwlock ,
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.Nm rw_init ,
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.Nm rw_init_flags ,
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.Nm rw_destroy ,
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.Nm rw_rlock ,
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.Nm rw_wlock ,
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.Nm rw_runlock ,
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.Nm rw_wunlock ,
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.Nm rw_unlock ,
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.Nm rw_try_rlock ,
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.Nm rw_try_upgrade ,
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.Nm rw_try_wlock ,
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.Nm rw_downgrade ,
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.Nm rw_sleep ,
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.Nm rw_initialized ,
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.Nm rw_wowned ,
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.Nm rw_assert ,
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.Nm RW_SYSINIT ,
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.Nm RW_SYSINIT_FLAGS
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.Nd kernel reader/writer lock
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/lock.h
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.In sys/rwlock.h
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.Ft void
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.Fn rw_init "struct rwlock *rw" "const char *name"
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.Ft void
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.Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts"
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.Ft void
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.Fn rw_destroy "struct rwlock *rw"
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.Ft void
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.Fn rw_rlock "struct rwlock *rw"
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.Ft void
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.Fn rw_wlock "struct rwlock *rw"
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.Ft int
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.Fn rw_try_rlock "struct rwlock *rw"
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.Ft int
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.Fn rw_try_wlock "struct rwlock *rw"
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.Ft void
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.Fn rw_runlock "struct rwlock *rw"
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.Ft void
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.Fn rw_wunlock "struct rwlock *rw"
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.Ft void
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.Fn rw_unlock "struct rwlock *rw"
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.Ft int
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.Fn rw_try_upgrade "struct rwlock *rw"
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.Ft void
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.Fn rw_downgrade "struct rwlock *rw"
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.Ft int
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.Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo"
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.Ft int
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.Fn rw_initialized "const struct rwlock *rw"
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.Ft int
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.Fn rw_wowned "const struct rwlock *rw"
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.Pp
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.Cd "options INVARIANTS"
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.Cd "options INVARIANT_SUPPORT"
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.Ft void
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.Fn rw_assert "const struct rwlock *rw" "int what"
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.In sys/kernel.h
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.Fn RW_SYSINIT "name" "struct rwlock *rw" "const char *desc"
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.Fn RW_SYSINIT_FLAGS "name" "struct rwlock *rw" "const char *desc" "int flags"
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.Sh DESCRIPTION
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Reader/writer locks allow shared access to protected data by multiple threads,
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or exclusive access by a single thread.
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The threads with shared access are known as
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.Em readers
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since they only read the protected data.
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A thread with exclusive access is known as a
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.Em writer
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since it can modify protected data.
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.Pp
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Although reader/writer locks look very similar to
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.Xr sx 9
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locks, their usage pattern is different.
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Reader/writer locks can be treated as mutexes (see
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.Xr mutex 9 )
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with shared/exclusive semantics.
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Unlike
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.Xr sx 9 ,
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an
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.Nm
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can be locked while holding a non-spin mutex, and an
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.Nm
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cannot be held while sleeping.
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The
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.Nm
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locks have priority propagation like mutexes, but priority
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can be propagated only to writers.
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This limitation comes from the fact that readers
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are anonymous.
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Another important property is that readers can always recurse,
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and exclusive locks can be made recursive selectively.
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.Ss Macros and Functions
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.Bl -tag -width indent
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.It Fn rw_init "struct rwlock *rw" "const char *name"
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Initialize structure located at
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.Fa rw
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as reader/writer lock, described by name
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.Fa name .
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The description is used solely for debugging purposes.
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This function must be called before any other operations
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on the lock.
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.It Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts"
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Initialize the rw lock just like the
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.Fn rw_init
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function, but specifying a set of optional flags to alter the
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behaviour of
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.Fa rw ,
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through the
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.Fa opts
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argument.
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It contains one or more of the following flags:
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.Bl -tag -width ".Dv RW_NOPROFILE"
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.It Dv RW_DUPOK
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Witness should not log messages about duplicate locks being acquired.
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.It Dv RW_NOPROFILE
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Do not profile this lock.
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.It Dv RW_NOWITNESS
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Instruct
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.Xr witness 4
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to ignore this lock.
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.It Dv RW_QUIET
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Do not log any operations for this lock via
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.Xr ktr 4 .
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.It Dv RW_RECURSE
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Allow threads to recursively acquire exclusive locks for
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.Fa rw .
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.It Dv RW_NEW
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If the kernel has been compiled with
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.Cd "option INVARIANTS" ,
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.Fn rw_init_flags
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will assert that the
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.Fa rw
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has not been initialized multiple times without intervening calls to
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.Fn rw_destroy
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unless this option is specified.
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.El
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.It Fn rw_rlock "struct rwlock *rw"
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Lock
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.Fa rw
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as a reader.
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If any thread holds this lock exclusively, the current thread blocks,
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and its priority is propagated to the exclusive holder.
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The
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.Fn rw_rlock
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function can be called when the thread has already acquired reader
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access on
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.Fa rw .
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This is called
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.Dq "recursing on a lock" .
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.It Fn rw_wlock "struct rwlock *rw"
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Lock
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.Fa rw
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as a writer.
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If there are any shared owners of the lock, the current thread blocks.
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The
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.Fn rw_wlock
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function can be called recursively only if
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.Fa rw
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has been initialized with the
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.Dv RW_RECURSE
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option enabled.
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.It Fn rw_try_rlock "struct rwlock *rw"
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Try to lock
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.Fa rw
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as a reader.
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This function will return true if the operation succeeds, otherwise 0
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will be returned.
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.It Fn rw_try_wlock "struct rwlock *rw"
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Try to lock
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.Fa rw
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as a writer.
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This function will return true if the operation succeeds, otherwise 0
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will be returned.
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.It Fn rw_runlock "struct rwlock *rw"
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This function releases a shared lock previously acquired by
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.Fn rw_rlock .
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.It Fn rw_wunlock "struct rwlock *rw"
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This function releases an exclusive lock previously acquired by
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.Fn rw_wlock .
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.It Fn rw_unlock "struct rwlock *rw"
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This function releases a shared lock previously acquired by
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.Fn rw_rlock
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or an exclusive lock previously acquired by
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.Fn rw_wlock .
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.It Fn rw_try_upgrade "struct rwlock *rw"
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Attempt to upgrade a single shared lock to an exclusive lock.
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The current thread must hold a shared lock of
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.Fa rw .
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This will only succeed if the current thread holds the only shared lock on
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.Fa rw ,
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and it only holds a single shared lock.
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If the attempt succeeds
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.Fn rw_try_upgrade
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will return a non-zero value,
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and the current thread will hold an exclusive lock.
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If the attempt fails
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.Fn rw_try_upgrade
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will return zero,
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and the current thread will still hold a shared lock.
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.It Fn rw_downgrade "struct rwlock *rw"
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Convert an exclusive lock into a single shared lock.
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The current thread must hold an exclusive lock of
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.Fa rw .
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.It Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo"
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Atomically release
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.Fa rw
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while waiting for an event.
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For more details on the parameters to this function,
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see
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.Xr sleep 9 .
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.It Fn rw_initialized "const struct rwlock *rw"
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This function returns non-zero if
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.Fa rw
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has been initialized, and zero otherwise.
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.It Fn rw_destroy "struct rwlock *rw"
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This functions destroys a lock previously initialized with
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.Fn rw_init .
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The
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.Fa rw
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lock must be unlocked.
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.It Fn rw_wowned "const struct rwlock *rw"
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This function returns a non-zero value if the current thread owns an
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exclusive lock on
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.Fa rw .
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.It Fn rw_assert "const struct rwlock *rw" "int what"
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This function allows assertions specified in
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.Fa what
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to be made about
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.Fa rw .
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If the assertions are not true and the kernel is compiled
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with
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.Cd "options INVARIANTS"
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and
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.Cd "options INVARIANT_SUPPORT" ,
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the kernel will panic.
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Currently the following base assertions are supported:
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.Bl -tag -width ".Dv RA_UNLOCKED"
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.It Dv RA_LOCKED
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Assert that current thread holds either a shared or exclusive lock
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of
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.Fa rw .
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.It Dv RA_RLOCKED
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Assert that current thread holds a shared lock of
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.Fa rw .
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.It Dv RA_WLOCKED
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Assert that current thread holds an exclusive lock of
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.Fa rw .
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.It Dv RA_UNLOCKED
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Assert that current thread holds neither a shared nor exclusive lock of
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.Fa rw .
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.El
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.Pp
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In addition, one of the following optional flags may be specified with
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.Dv RA_LOCKED ,
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.Dv RA_RLOCKED ,
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or
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.Dv RA_WLOCKED :
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.Bl -tag -width ".Dv RA_NOTRECURSED"
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.It Dv RA_RECURSED
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Assert that the current thread holds a recursive lock of
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.Fa rw .
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.It Dv RA_NOTRECURSED
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Assert that the current thread does not hold a recursive lock of
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.Fa rw .
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.El
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.El
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.Sh SEE ALSO
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.Xr locking 9 ,
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.Xr mutex 9 ,
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.Xr panic 9 ,
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.Xr sema 9 ,
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.Xr sx 9
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.Sh HISTORY
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These
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functions appeared in
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.Fx 7.0 .
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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facility was written by
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.An "John Baldwin" .
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This manual page was written by
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.An "Gleb Smirnoff" .
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.Sh BUGS
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A kernel without
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.Dv WITNESS
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cannot assert whether the current thread does or does not hold a read lock.
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.Dv RA_LOCKED
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and
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.Dv RA_RLOCKED
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can only assert that
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.Em any
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thread holds a read lock.
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They cannot ensure that the current thread holds a read lock.
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Further,
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.Dv RA_UNLOCKED
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can only assert that the current thread does not hold a write lock.
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.Pp
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Reader/writer is a bit of an awkward name.
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An
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.Nm
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can also be called a
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.Dq Robert Watson
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lock if desired.
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