1ae1c2a3bd
Introduce for this operation the reverse NO_ADAPTIVE_SX option. The flag SX_ADAPTIVESPIN to be passed to sx_init_flags(9) gets suppressed and the new flag, offering the reversed logic, SX_NOADAPTIVE is added. Additively implements adaptive spininning for sx held in shared mode. The spinning limit can be handled through sysctls in order to be tuned while the code doesn't reach the release, after which time they should be dropped probabilly. This change has made been necessary by recent benchmarks where it does improve concurrency of workloads in presence of high contention (ie. ZFS). KPI breakage is documented by __FreeBSD_version bumping, manpage and UPDATING updates. Requested by: jeff, kmacy Reviewed by: jeff Tested by: pho
329 lines
8.7 KiB
Groff
329 lines
8.7 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 May 28, 2009
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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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.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 "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 "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 *description"
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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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.Bl -tag -width SX_NOADAPTIVE
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.It Dv SX_NOADAPTIVE
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If the kernel is not compiled with
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.Cd "options NO_ADAPTIVE_SX" ,
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then lock operations for
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.Fa sx
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will spin instead of sleeping while an exclusive lock holder is executing on
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another CPU.
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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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.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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.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 locking 9 ,
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.Xr lock 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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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 SA_LOCKED
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and
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.Dv SA_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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