234 lines
6.4 KiB
Groff
234 lines
6.4 KiB
Groff
.\"
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.\" Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. 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(s), this list of conditions and the following disclaimer as
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.\" the first lines of this file unmodified other than the possible
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.\" addition of one or more copyright notices.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice(s), 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 COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
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.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
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.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
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.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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.\" 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 SUCH
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.\" DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd February 1, 2006
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.Dt SX 9
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.Os
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.Sh NAME
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.Nm sx ,
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.Nm sx_init ,
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.Nm sx_destroy ,
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.Nm sx_slock ,
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.Nm sx_xlock ,
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.Nm sx_try_slock ,
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.Nm sx_try_xlock ,
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.Nm sx_sunlock ,
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.Nm sx_xunlock ,
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.Nm sx_try_upgrade ,
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.Nm sx_downgrade ,
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.Nm sx_assert ,
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.Nm sx_unlock ,
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.Nm sx_xlocked ,
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.Nm SX_SYSINIT
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.Nd kernel shared/exclusive 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/sx.h
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.Ft void
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.Fn sx_init "struct sx *sx" "const char *description"
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.Ft void
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.Fn sx_destroy "struct sx *sx"
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.Ft void
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.Fn sx_slock "struct sx *sx"
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.Ft void
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.Fn sx_xlock "struct sx *sx"
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.Ft int
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.Fn sx_try_slock "struct sx *sx"
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.Ft int
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.Fn sx_try_xlock "struct sx *sx"
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.Ft void
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.Fn sx_sunlock "struct sx *sx"
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.Ft void
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.Fn sx_xunlock "struct sx *sx"
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.Ft int
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.Fn sx_try_upgrade "struct sx *sx"
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.Ft void
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.Fn sx_downgrade "struct sx *sx"
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.Ft void
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.Fn sx_assert "struct sx *sx" "int what"
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.Ft int
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.Fn sx_xlocked "struct sx *sx"
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.\"
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.Ss Nm Ss utility macros
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.Fn sx_unlock "struct sx *sx"
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.Fn SX_SYSINIT "name" "struct sx *sx" "const char *description"
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.\"
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.Ss Kernel options
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.Cd "options INVARIANTS"
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.Cd "options INVARIANT_SUPPORT"
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.Sh DESCRIPTION
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Shared/exclusive locks are used to protect data that are read far more often
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than they are written.
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Mutexes are inherently more efficient than shared/exclusive locks, so
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shared/exclusive locks should be used prudently.
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.Pp
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Shared/exclusive locks are created with
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.Fn sx_init ,
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where
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.Fa sx
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is a pointer to space for a
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.Vt struct sx ,
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and
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.Fa description
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is a pointer to a null-terminated character string that describes the
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shared/exclusive lock.
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Shared/exclusive locks are destroyed with
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.Fn sx_destroy .
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Threads acquire and release a shared lock by calling
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.Fn sx_slock
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or
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.Fn sx_try_slock
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and
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.Fn sx_sunlock
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or
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.Fn sx_unlock .
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Threads acquire and release an exclusive lock by calling
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.Fn sx_xlock
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or
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.Fn sx_try_xlock
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and
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.Fn sx_xunlock
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or
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.Fn sx_unlock .
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A thread can attempt to upgrade a currently held shared lock to an exclusive
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lock by calling
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.Fn sx_try_upgrade .
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A thread that has an exclusive lock can downgrade it to a shared lock by
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calling
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.Fn sx_downgrade .
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.Pp
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.Fn sx_try_slock
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and
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.Fn sx_try_xlock
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will return 0 if the shared/exclusive lock cannot be acquired immediately;
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otherwise the shared/exclusive lock will be acquired and a non-zero value will
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be returned.
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.Pp
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.Fn sx_try_upgrade
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will return 0 if the shared lock cannot be upgraded to an exclusive lock
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immediately; otherwise the exclusive lock will be acquired and a non-zero value
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will be returned.
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.Pp
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When compiled 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
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.Fn sx_assert
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function tests
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.Fa sx
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for the assertions specified in
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.Fa what ,
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and panics if they are not met.
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The following assertions are supported:
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.Bl -tag -width ".Dv SX_UNLOCKED"
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.It Dv SX_LOCKED
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Assert that the current thread has either a shared or an exclusive lock on the
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.Vt sx
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lock pointed to by the first argument.
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.It Dv SX_SLOCKED
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Assert that the current thread has a shared lock on the
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.Vt sx
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lock pointed to by
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the first argument.
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.It Dv SX_XLOCKED
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Assert that the current thread has an exclusive lock on the
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.Vt sx
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lock pointed to
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by the first argument.
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.It Dv SX_UNLOCKED
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Assert that the current thread has no lock on the
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.Vt sx
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lock pointed to
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by the first argument.
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.El
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.Pp
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.Fn sx_xlocked
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will return non-zero if the current process holds the exclusive lock;
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otherwise, it will return zero.
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.Pp
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For ease of programming,
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.Fn sx_unlock
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is provided as a macro frontend to the respective functions,
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.Fn sx_sunlock
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and
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.Fn sx_xunlock .
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Algorithms that are aware of what state the lock is in should use either
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of the two specific functions for a minor performance benefit.
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.Pp
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The
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.Fn SX_SYSINIT
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macro is used to generate a call to the
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.Fn sx_sysinit
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routine at system startup in order to initialize a given
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.Fa sx
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lock.
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The parameters are the same as
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.Fn sx_init
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but with an additional argument,
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.Fa name ,
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that is used in generating unique variable names for the related
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structures associated with the lock and the sysinit routine.
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.Pp
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A thread may not hold both a shared lock and an exclusive lock on the same
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lock simultaneously;
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attempting to do so will result in deadlock.
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.Sh CONTEXT
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A thread may hold a shared or exclusive lock on an
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.Nm
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lock while sleeping.
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As a result, an
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.Nm
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lock may not be acquired while holding a mutex.
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Otherwise, if one thread slept while holding an
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.Nm
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lock while another thread blocked on the same
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.Nm
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lock after acquiring a mutex, then the second thread would effectively
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end up sleeping while holding a mutex, which is not allowed.
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.Sh SEE ALSO
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.Xr mutex 9 ,
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.Xr panic 9 ,
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.Xr rwlock 9 ,
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.Xr sema 9
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.Sh BUGS
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Currently there is no way to assert that a lock is not held.
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This is not possible in the
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.No non- Ns Dv WITNESS
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case for asserting that this thread
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does not hold a shared lock.
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In the
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.No non- Ns Dv WITNESS
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case, the
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.Dv SX_LOCKED
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and
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.Dv SX_SLOCKED
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assertions merely check that some thread holds a shared lock.
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They do not ensure that the current thread holds a shared lock.
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