338 lines
8.9 KiB
Groff
338 lines
8.9 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 November 11, 2017
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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_init_flags ,
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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_slock_sig ,
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.Nm sx_xlock_sig ,
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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_unlock ,
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.Nm sx_try_upgrade ,
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.Nm sx_downgrade ,
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.Nm sx_sleep ,
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.Nm sx_xholder ,
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.Nm sx_xlocked ,
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.Nm sx_assert ,
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.Nm SX_SYSINIT ,
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.Nm SX_SYSINIT_FLAGS
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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_init_flags "struct sx *sx" "const char *description" "int opts"
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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_slock_sig "struct sx *sx"
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.Ft int
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.Fn sx_xlock_sig "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 void
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.Fn sx_unlock "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 int
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.Fn sx_sleep "void *chan" "struct sx *sx" "int priority" "const char *wmesg" "int timo"
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.Ft "struct thread *"
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.Fn sx_xholder "struct sx *sx"
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.Ft int
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.Fn sx_xlocked "const struct sx *sx"
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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 sx_assert "const struct sx *sx" "int what"
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.In sys/kernel.h
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.Fn SX_SYSINIT "name" "struct sx *sx" "const char *desc"
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.Fn SX_SYSINIT_FLAGS "name" "struct sx *sx" "const char *desc" "int flags"
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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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Shared/exclusive locks do not implement priority propagation like mutexes and
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reader/writer locks to prevent priority inversions, 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 either
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.Fn sx_init
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or
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.Fn sx_init_flags
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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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The
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.Fa opts
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argument to
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.Fn sx_init_flags
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specifies a set of optional flags to alter the behavior of
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.Fa sx .
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It contains one or more of the following flags:
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.It Dv SX_DUPOK
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Witness should not log messages about duplicate locks being acquired.
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.It Dv SX_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 SX_NOPROFILE
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Do not profile this lock.
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.It Dv SX_RECURSE
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Allow threads to recursively acquire exclusive locks for
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.Fa sx .
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.It Dv SX_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 SX_NEW
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If the kernel has been compiled with
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.Cd "options INVARIANTS" ,
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.Fn sx_init
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will assert that the
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.Fa sx
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has not been initialized multiple times without intervening calls to
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.Fn sx_destroy
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unless this option is specified.
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.El
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.Pp
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Shared/exclusive locks are destroyed with
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.Fn sx_destroy .
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The lock
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.Fa sx
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must not be locked by any thread when it is destroyed.
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.Pp
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Threads acquire and release a shared lock by calling
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.Fn sx_slock ,
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.Fn sx_slock_sig
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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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.Fn sx_xlock_sig
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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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.Fn sx_slock_sig
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and
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.Fn sx_xlock_sig
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do the same as their normal versions but performing an interruptible sleep.
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They return a non-zero value if the sleep has been interrupted by a signal
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or an interrupt, otherwise 0.
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.Pp
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A thread can atomically release a shared/exclusive lock while waiting for an
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event by calling
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.Fn sx_sleep .
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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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.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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One of the following assertions must be specified:
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.Bl -tag -width ".Dv SA_UNLOCKED"
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.It Dv SA_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 SA_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 SA_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 SA_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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In addition, one of the following optional assertions may be included with
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either an
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.Dv SA_LOCKED ,
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.Dv SA_SLOCKED ,
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or
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.Dv SA_XLOCKED
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assertion:
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.Bl -tag -width ".Dv SA_NOTRECURSED"
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.It Dv SA_RECURSED
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Assert that the current thread has a recursed lock on
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.Fa sx .
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.It Dv SA_NOTRECURSED
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Assert that the current thread does not have a recursed lock on
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.Fa sx .
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.El
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.Pp
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.Fn sx_xholder
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will return a pointer to the thread which currently holds an exclusive lock on
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.Fa sx .
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If no thread holds an exclusive lock on
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.Fa sx ,
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then
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.Dv NULL
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is returned instead.
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.Pp
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.Fn sx_xlocked
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will return non-zero if the current thread 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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The
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.Fn SX_SYSINIT_FLAGS
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macro can similarly be used to initialize a given
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.Fa sx
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lock using
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.Fn sx_init_flags .
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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 lock 9 ,
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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 rwlock 9 ,
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.Xr sema 9
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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 shared lock.
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.Dv SA_LOCKED
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and
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.Dv SA_SLOCKED
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can only assert that
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.Em any
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thread holds a shared lock.
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They cannot ensure that the current thread holds a shared lock.
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Further,
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.Dv SA_UNLOCKED
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can only assert that the current thread does not hold an exclusive lock.
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