2001-07-14 19:41:16 +00:00
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.\"
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2001-08-13 16:33:00 +00:00
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.\" Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. All rights reserved.
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2001-07-14 19:41:16 +00:00
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.\"
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2001-03-05 19:59:41 +00:00
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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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2001-07-14 19:41:16 +00:00
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.\" the first lines of this file unmodified other than the possible
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2001-03-05 19:59:41 +00:00
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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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2017-11-21 14:59:23 +00:00
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.Dd November 11, 2017
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2001-03-05 19:59:41 +00:00
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.Dt SX 9
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.Os
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.Sh NAME
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|
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.Nm sx ,
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.Nm sx_init ,
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2007-03-31 23:23:42 +00:00
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.Nm sx_init_flags ,
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2001-03-05 19:59:41 +00:00
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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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2007-11-25 01:55:53 +00:00
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.Nm sx_slock_sig ,
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.Nm sx_xlock_sig ,
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2001-08-07 04:29:53 +00:00
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.Nm sx_try_slock ,
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.Nm sx_try_xlock ,
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2001-03-05 19:59:41 +00:00
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.Nm sx_sunlock ,
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2001-08-14 05:10:07 +00:00
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.Nm sx_xunlock ,
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2007-03-09 16:52:26 +00:00
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.Nm sx_unlock ,
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2001-08-14 05:10:07 +00:00
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.Nm sx_try_upgrade ,
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2001-10-23 22:51:59 +00:00
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.Nm sx_downgrade ,
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2007-03-09 22:41:01 +00:00
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.Nm sx_sleep ,
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2007-05-19 20:24:32 +00:00
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.Nm sx_xholder ,
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2006-07-12 15:40:35 +00:00
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.Nm sx_xlocked ,
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2007-03-09 16:52:26 +00:00
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.Nm sx_assert ,
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2017-11-21 14:59:23 +00:00
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.Nm SX_SYSINIT ,
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.Nm SX_SYSINIT_FLAGS
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2001-04-04 12:00:29 +00:00
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.Nd kernel shared/exclusive lock
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2001-03-05 19:59:41 +00:00
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.Sh SYNOPSIS
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2001-10-04 09:01:49 +00:00
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.In sys/param.h
|
2001-10-01 16:09:29 +00:00
|
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.In sys/lock.h
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.In sys/sx.h
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2001-03-05 19:59:41 +00:00
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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
|
2007-03-31 23:23:42 +00:00
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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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2001-03-05 19:59:41 +00:00
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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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2001-08-07 04:29:53 +00:00
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.Ft int
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2007-11-25 01:55:53 +00:00
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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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2001-08-07 04:29:53 +00:00
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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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2001-03-05 19:59:41 +00:00
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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"
|
2007-03-09 16:52:26 +00:00
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.Ft void
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.Fn sx_unlock "struct sx *sx"
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2001-08-13 21:25:30 +00:00
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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"
|
2006-07-12 15:40:35 +00:00
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.Ft int
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2007-03-09 22:41:01 +00:00
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.Fn sx_sleep "void *chan" "struct sx *sx" "int priority" "const char *wmesg" "int timo"
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2007-05-19 20:24:32 +00:00
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.Ft "struct thread *"
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.Fn sx_xholder "struct sx *sx"
|
2007-03-09 22:41:01 +00:00
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.Ft int
|
2011-11-16 21:51:17 +00:00
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.Fn sx_xlocked "const struct sx *sx"
|
2007-03-09 16:52:26 +00:00
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.Pp
|
2004-07-11 16:08:25 +00:00
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.Cd "options INVARIANTS"
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.Cd "options INVARIANT_SUPPORT"
|
2007-03-09 16:52:26 +00:00
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.Ft void
|
2011-11-16 21:51:17 +00:00
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.Fn sx_assert "const struct sx *sx" "int what"
|
2007-03-09 16:52:26 +00:00
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|
.In sys/kernel.h
|
2017-11-21 14:59:23 +00:00
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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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2001-03-05 19:59:41 +00:00
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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.
|
2007-03-31 23:23:42 +00:00
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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
|
2001-03-05 19:59:41 +00:00
|
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shared/exclusive locks should be used prudently.
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.Pp
|
2007-03-31 23:23:42 +00:00
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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
|
2001-03-05 19:59:41 +00:00
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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.
|
2007-03-31 23:23:42 +00:00
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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:
|
2020-09-05 20:22:02 +00:00
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.Bl -tag -width SX_NOWITNESS
|
2007-03-31 23:23:42 +00:00
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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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2007-05-19 16:38:10 +00:00
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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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2007-03-31 23:23:42 +00:00
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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 .
|
2014-12-13 21:00:10 +00:00
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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.
|
2007-03-31 23:23:42 +00:00
|
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.El
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.Pp
|
2001-03-05 19:59:41 +00:00
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Shared/exclusive locks are destroyed with
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.Fn sx_destroy .
|
2007-03-31 23:23:42 +00:00
|
|
|
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
|
2001-03-05 19:59:41 +00:00
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Threads acquire and release a shared lock by calling
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2007-11-25 01:55:53 +00:00
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.Fn sx_slock ,
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.Fn sx_slock_sig
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2001-08-07 04:29:53 +00:00
|
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or
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.Fn sx_try_slock
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2001-03-05 19:59:41 +00:00
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and
|
2004-07-11 16:08:25 +00:00
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.Fn sx_sunlock
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or
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.Fn sx_unlock .
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2001-03-05 19:59:41 +00:00
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Threads acquire and release an exclusive lock by calling
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2007-11-25 01:55:53 +00:00
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.Fn sx_xlock ,
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.Fn sx_xlock_sig
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2001-08-07 04:29:53 +00:00
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or
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.Fn sx_try_xlock
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2001-03-05 19:59:41 +00:00
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and
|
2004-07-11 16:08:25 +00:00
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.Fn sx_xunlock
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or
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|
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.Fn sx_unlock .
|
2004-02-24 21:36:39 +00:00
|
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|
A thread can attempt to upgrade a currently held shared lock to an exclusive
|
2001-08-13 21:25:30 +00:00
|
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|
lock by calling
|
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|
|
.Fn sx_try_upgrade .
|
2004-02-24 21:36:39 +00:00
|
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|
A thread that has an exclusive lock can downgrade it to a shared lock by
|
2001-08-13 21:25:30 +00:00
|
|
|
calling
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|
.Fn sx_downgrade .
|
2002-04-17 21:26:43 +00:00
|
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.Pp
|
2001-08-07 04:29:53 +00:00
|
|
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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
|
2001-08-13 21:25:30 +00:00
|
|
|
.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
|
2007-11-25 01:55:53 +00:00
|
|
|
.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
|
2007-03-09 22:41:01 +00:00
|
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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,
|
|
|
|
see
|
|
|
|
.Xr sleep 9 .
|
|
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|
.Pp
|
2004-01-01 05:16:35 +00:00
|
|
|
When compiled with
|
|
|
|
.Cd "options INVARIANTS"
|
|
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|
and
|
|
|
|
.Cd "options INVARIANT_SUPPORT" ,
|
|
|
|
the
|
2001-10-23 22:51:59 +00:00
|
|
|
.Fn sx_assert
|
2004-01-01 05:16:35 +00:00
|
|
|
function tests
|
2004-06-16 08:33:57 +00:00
|
|
|
.Fa sx
|
2004-01-01 05:16:35 +00:00
|
|
|
for the assertions specified in
|
2004-06-16 08:33:57 +00:00
|
|
|
.Fa what ,
|
2004-01-01 05:16:35 +00:00
|
|
|
and panics if they are not met.
|
2007-03-31 23:23:42 +00:00
|
|
|
One of the following assertions must be specified:
|
2007-05-19 21:26:05 +00:00
|
|
|
.Bl -tag -width ".Dv SA_UNLOCKED"
|
|
|
|
.It Dv SA_LOCKED
|
2001-10-23 22:51:59 +00:00
|
|
|
Assert that the current thread has either a shared or an exclusive lock on the
|
2001-11-21 11:44:36 +00:00
|
|
|
.Vt sx
|
|
|
|
lock pointed to by the first argument.
|
2007-05-19 21:26:05 +00:00
|
|
|
.It Dv SA_SLOCKED
|
2001-11-21 11:44:36 +00:00
|
|
|
Assert that the current thread has a shared lock on the
|
|
|
|
.Vt sx
|
|
|
|
lock pointed to by
|
2001-10-23 22:51:59 +00:00
|
|
|
the first argument.
|
2007-05-19 21:26:05 +00:00
|
|
|
.It Dv SA_XLOCKED
|
2001-11-21 11:44:36 +00:00
|
|
|
Assert that the current thread has an exclusive lock on the
|
|
|
|
.Vt sx
|
|
|
|
lock pointed to
|
2001-10-23 22:51:59 +00:00
|
|
|
by the first argument.
|
2007-05-19 21:26:05 +00:00
|
|
|
.It Dv SA_UNLOCKED
|
2004-02-04 08:24:05 +00:00
|
|
|
Assert that the current thread has no lock on the
|
|
|
|
.Vt sx
|
|
|
|
lock pointed to
|
|
|
|
by the first argument.
|
2001-10-23 22:51:59 +00:00
|
|
|
.El
|
|
|
|
.Pp
|
2007-03-31 23:23:42 +00:00
|
|
|
In addition, one of the following optional assertions may be included with
|
|
|
|
either an
|
2007-05-19 21:26:05 +00:00
|
|
|
.Dv SA_LOCKED ,
|
|
|
|
.Dv SA_SLOCKED ,
|
2007-03-31 23:23:42 +00:00
|
|
|
or
|
2007-05-19 21:26:05 +00:00
|
|
|
.Dv SA_XLOCKED
|
2007-03-31 23:23:42 +00:00
|
|
|
assertion:
|
2007-05-19 21:26:05 +00:00
|
|
|
.Bl -tag -width ".Dv SA_NOTRECURSED"
|
|
|
|
.It Dv SA_RECURSED
|
2007-03-31 23:23:42 +00:00
|
|
|
Assert that the current thread has a recursed lock on
|
|
|
|
.Fa sx .
|
2007-05-19 21:26:05 +00:00
|
|
|
.It Dv SA_NOTRECURSED
|
2007-03-31 23:23:42 +00:00
|
|
|
Assert that the current thread does not have a recursed lock on
|
|
|
|
.Fa sx .
|
|
|
|
.El
|
|
|
|
.Pp
|
2007-05-19 20:24:32 +00:00
|
|
|
.Fn sx_xholder
|
|
|
|
will return a pointer to the thread which currently holds an exclusive lock on
|
|
|
|
.Fa sx .
|
|
|
|
If no thread holds an exclusive lock on
|
|
|
|
.Fa sx ,
|
|
|
|
then
|
|
|
|
.Dv NULL
|
|
|
|
is returned instead.
|
|
|
|
.Pp
|
2006-07-12 15:40:35 +00:00
|
|
|
.Fn sx_xlocked
|
2007-03-09 22:41:01 +00:00
|
|
|
will return non-zero if the current thread holds the exclusive lock;
|
2006-07-12 15:40:35 +00:00
|
|
|
otherwise, it will return zero.
|
|
|
|
.Pp
|
2004-07-11 16:08:25 +00:00
|
|
|
For ease of programming,
|
|
|
|
.Fn sx_unlock
|
|
|
|
is provided as a macro frontend to the respective functions,
|
|
|
|
.Fn sx_sunlock
|
|
|
|
and
|
|
|
|
.Fn sx_xunlock .
|
|
|
|
Algorithms that are aware of what state the lock is in should use either
|
|
|
|
of the two specific functions for a minor performance benefit.
|
|
|
|
.Pp
|
2003-12-27 14:01:08 +00:00
|
|
|
The
|
|
|
|
.Fn SX_SYSINIT
|
|
|
|
macro is used to generate a call to the
|
|
|
|
.Fn sx_sysinit
|
|
|
|
routine at system startup in order to initialize a given
|
|
|
|
.Fa sx
|
|
|
|
lock.
|
|
|
|
The parameters are the same as
|
|
|
|
.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
|
|
|
|
structures associated with the lock and the sysinit routine.
|
2017-11-21 14:59:23 +00:00
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|
The
|
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.Fn SX_SYSINIT_FLAGS
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|
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|
macro can similarly be used to initialize a given
|
|
|
|
.Fa sx
|
|
|
|
lock using
|
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|
|
.Fn sx_init_flags .
|
2003-12-27 14:01:08 +00:00
|
|
|
.Pp
|
2004-02-24 21:36:39 +00:00
|
|
|
A thread may not hold both a shared lock and an exclusive lock on the same
|
|
|
|
lock simultaneously;
|
2001-03-05 19:59:41 +00:00
|
|
|
attempting to do so will result in deadlock.
|
2003-06-12 09:06:25 +00:00
|
|
|
.Sh CONTEXT
|
2004-02-24 21:36:39 +00:00
|
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|
A thread may hold a shared or exclusive lock on an
|
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|
|
.Nm
|
|
|
|
lock while sleeping.
|
2005-01-05 22:04:12 +00:00
|
|
|
As a result, an
|
|
|
|
.Nm
|
|
|
|
lock may not be acquired while holding a mutex.
|
|
|
|
Otherwise, if one thread slept while holding an
|
|
|
|
.Nm
|
|
|
|
lock while another thread blocked on the same
|
|
|
|
.Nm
|
|
|
|
lock after acquiring a mutex, then the second thread would effectively
|
|
|
|
end up sleeping while holding a mutex, which is not allowed.
|
2001-03-05 19:59:41 +00:00
|
|
|
.Sh SEE ALSO
|
2007-03-30 18:07:26 +00:00
|
|
|
.Xr lock 9 ,
|
2014-12-15 20:48:06 +00:00
|
|
|
.Xr locking 9 ,
|
2001-09-17 07:24:48 +00:00
|
|
|
.Xr mutex 9 ,
|
2004-01-01 05:16:35 +00:00
|
|
|
.Xr panic 9 ,
|
2006-02-01 20:30:55 +00:00
|
|
|
.Xr rwlock 9 ,
|
2001-09-17 07:24:48 +00:00
|
|
|
.Xr sema 9
|
2001-10-23 22:51:59 +00:00
|
|
|
.Sh BUGS
|
2016-03-29 16:07:51 +00:00
|
|
|
A kernel without
|
|
|
|
.Dv WITNESS
|
|
|
|
cannot assert whether the current thread does or does not hold a shared lock.
|
2007-05-19 21:26:05 +00:00
|
|
|
.Dv SA_LOCKED
|
2001-10-23 22:51:59 +00:00
|
|
|
and
|
2007-05-19 21:26:05 +00:00
|
|
|
.Dv SA_SLOCKED
|
2016-03-29 16:07:51 +00:00
|
|
|
can only assert that
|
|
|
|
.Em any
|
|
|
|
thread holds a shared lock.
|
|
|
|
They cannot ensure that the current thread holds a shared lock.
|
|
|
|
Further,
|
|
|
|
.Dv SA_UNLOCKED
|
|
|
|
can only assert that the current thread does not hold an exclusive lock.
|