b0b93a3c55
obtaining and releasing shared and exclusive locks. The algorithms for manipulating the lock cookie are very similar to that rwlocks. This patch also adds support for exclusive locks using the same algorithm as mutexes. A new sx_init_flags() function has been added so that optional flags can be specified to alter a given locks behavior. The flags include SX_DUPOK, SX_NOWITNESS, SX_NOPROFILE, and SX_QUITE which are all identical in nature to the similar flags for mutexes. Adaptive spinning on select locks may be enabled by enabling the ADAPTIVE_SX kernel option. Only locks initialized with the SX_ADAPTIVESPIN flag via sx_init_flags() will adaptively spin. The common cases for sx_slock(), sx_sunlock(), sx_xlock(), and sx_xunlock() are now performed inline in non-debug kernels. As a result, <sys/sx.h> now requires <sys/lock.h> to be included prior to <sys/sx.h>. The new kernel option SX_NOINLINE can be used to disable the aforementioned inlining in non-debug kernels. The size of struct sx has changed, so the kernel ABI is probably greatly disturbed. MFC after: 1 month Submitted by: attilio Tested by: kris, pjd
299 lines
8.0 KiB
Groff
299 lines
8.0 KiB
Groff
.\"
|
|
.\" Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice(s), this list of conditions and the following disclaimer as
|
|
.\" the first lines of this file unmodified other than the possible
|
|
.\" addition of one or more copyright notices.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice(s), this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
|
|
.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
|
|
.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
|
|
.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
|
|
.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
|
|
.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
|
|
.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
|
|
.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
|
|
.\" DAMAGE.
|
|
.\"
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd February 1, 2006
|
|
.Dt SX 9
|
|
.Os
|
|
.Sh NAME
|
|
.Nm sx ,
|
|
.Nm sx_init ,
|
|
.Nm sx_init_flags ,
|
|
.Nm sx_destroy ,
|
|
.Nm sx_slock ,
|
|
.Nm sx_xlock ,
|
|
.Nm sx_try_slock ,
|
|
.Nm sx_try_xlock ,
|
|
.Nm sx_sunlock ,
|
|
.Nm sx_xunlock ,
|
|
.Nm sx_unlock ,
|
|
.Nm sx_try_upgrade ,
|
|
.Nm sx_downgrade ,
|
|
.Nm sx_sleep ,
|
|
.Nm sx_xlocked ,
|
|
.Nm sx_assert ,
|
|
.Nm SX_SYSINIT
|
|
.Nd kernel shared/exclusive lock
|
|
.Sh SYNOPSIS
|
|
.In sys/param.h
|
|
.In sys/lock.h
|
|
.In sys/sx.h
|
|
.Ft void
|
|
.Fn sx_init "struct sx *sx" "const char *description"
|
|
.Ft void
|
|
.Fn sx_init_flags "struct sx *sx" "const char *description" "int opts"
|
|
.Ft void
|
|
.Fn sx_destroy "struct sx *sx"
|
|
.Ft void
|
|
.Fn sx_slock "struct sx *sx"
|
|
.Ft void
|
|
.Fn sx_xlock "struct sx *sx"
|
|
.Ft int
|
|
.Fn sx_try_slock "struct sx *sx"
|
|
.Ft int
|
|
.Fn sx_try_xlock "struct sx *sx"
|
|
.Ft void
|
|
.Fn sx_sunlock "struct sx *sx"
|
|
.Ft void
|
|
.Fn sx_xunlock "struct sx *sx"
|
|
.Ft void
|
|
.Fn sx_unlock "struct sx *sx"
|
|
.Ft int
|
|
.Fn sx_try_upgrade "struct sx *sx"
|
|
.Ft void
|
|
.Fn sx_downgrade "struct sx *sx"
|
|
.Ft int
|
|
.Fn sx_sleep "void *chan" "struct sx *sx" "int priority" "const char *wmesg" "int timo"
|
|
.Ft int
|
|
.Fn sx_xlocked "struct sx *sx"
|
|
.Pp
|
|
.Cd "options INVARIANTS"
|
|
.Cd "options INVARIANT_SUPPORT"
|
|
.Ft void
|
|
.Fn sx_assert "struct sx *sx" "int what"
|
|
.In sys/kernel.h
|
|
.Fn SX_SYSINIT "name" "struct sx *sx" "const char *description"
|
|
.Sh DESCRIPTION
|
|
Shared/exclusive locks are used to protect data that are read far more often
|
|
than they are written.
|
|
Shared/exclusive locks do not implement priority propagation like mutexes and
|
|
reader/writer locks to prevent priority inversions, so
|
|
shared/exclusive locks should be used prudently.
|
|
.Pp
|
|
Shared/exclusive locks are created with either
|
|
.Fn sx_init
|
|
or
|
|
.Fn sx_init_flags
|
|
where
|
|
.Fa sx
|
|
is a pointer to space for a
|
|
.Vt struct sx ,
|
|
and
|
|
.Fa description
|
|
is a pointer to a null-terminated character string that describes the
|
|
shared/exclusive lock.
|
|
The
|
|
.Fa opts
|
|
argument to
|
|
.Fn sx_init_flags
|
|
specifies a set of optional flags to alter the behavior of
|
|
.Fa sx .
|
|
It contains one or more of the following flags:
|
|
.Bl -tag -width SX_ADAPTIVESPIN
|
|
.It Dv SX_ADAPTIVESPIN
|
|
If the kernel is compiled with
|
|
.Cd "options ADAPTIVE_SX" ,
|
|
then lock operations for
|
|
.Fa sx
|
|
will spin instead of sleeping while an exclusive lock holder is executing on
|
|
another CPU.
|
|
.It Dv SX_DUPOK
|
|
Witness should not log messages about duplicate locks being acquired.
|
|
.It Dv SX_NOWITNESS
|
|
Instruct
|
|
.Xr witness 4
|
|
to ignore this lock.
|
|
.It Dv SX_NOPROFILE
|
|
Do not profile this lock.
|
|
.It Dv SX_QUIET
|
|
Do not log any operations for this lock via
|
|
.Xr ktr 4 .
|
|
.El
|
|
.Pp
|
|
Shared/exclusive locks are destroyed with
|
|
.Fn sx_destroy .
|
|
The lock
|
|
.Fa sx
|
|
must not be locked by any thread when it is destroyed.
|
|
.Pp
|
|
Threads acquire and release a shared lock by calling
|
|
.Fn sx_slock
|
|
or
|
|
.Fn sx_try_slock
|
|
and
|
|
.Fn sx_sunlock
|
|
or
|
|
.Fn sx_unlock .
|
|
Threads acquire and release an exclusive lock by calling
|
|
.Fn sx_xlock
|
|
or
|
|
.Fn sx_try_xlock
|
|
and
|
|
.Fn sx_xunlock
|
|
or
|
|
.Fn sx_unlock .
|
|
A thread can attempt to upgrade a currently held shared lock to an exclusive
|
|
lock by calling
|
|
.Fn sx_try_upgrade .
|
|
A thread that has an exclusive lock can downgrade it to a shared lock by
|
|
calling
|
|
.Fn sx_downgrade .
|
|
.Pp
|
|
.Fn sx_try_slock
|
|
and
|
|
.Fn sx_try_xlock
|
|
will return 0 if the shared/exclusive lock cannot be acquired immediately;
|
|
otherwise the shared/exclusive lock will be acquired and a non-zero value will
|
|
be returned.
|
|
.Pp
|
|
.Fn sx_try_upgrade
|
|
will return 0 if the shared lock cannot be upgraded to an exclusive lock
|
|
immediately; otherwise the exclusive lock will be acquired and a non-zero value
|
|
will be returned.
|
|
.Pp
|
|
A thread can atomically release a shared/exclusive lock while waiting for an
|
|
event by calling
|
|
.Fn sx_sleep .
|
|
For more details on the parameters to this function,
|
|
see
|
|
.Xr sleep 9 .
|
|
.Pp
|
|
When compiled with
|
|
.Cd "options INVARIANTS"
|
|
and
|
|
.Cd "options INVARIANT_SUPPORT" ,
|
|
the
|
|
.Fn sx_assert
|
|
function tests
|
|
.Fa sx
|
|
for the assertions specified in
|
|
.Fa what ,
|
|
and panics if they are not met.
|
|
One of the following assertions must be specified:
|
|
.Bl -tag -width ".Dv SX_UNLOCKED"
|
|
.It Dv SX_LOCKED
|
|
Assert that the current thread has either a shared or an exclusive lock on the
|
|
.Vt sx
|
|
lock pointed to by the first argument.
|
|
.It Dv SX_SLOCKED
|
|
Assert that the current thread has a shared lock on the
|
|
.Vt sx
|
|
lock pointed to by
|
|
the first argument.
|
|
.It Dv SX_XLOCKED
|
|
Assert that the current thread has an exclusive lock on the
|
|
.Vt sx
|
|
lock pointed to
|
|
by the first argument.
|
|
.It Dv SX_UNLOCKED
|
|
Assert that the current thread has no lock on the
|
|
.Vt sx
|
|
lock pointed to
|
|
by the first argument.
|
|
.El
|
|
.Pp
|
|
In addition, one of the following optional assertions may be included with
|
|
either an
|
|
.Dv SX_LOCKED ,
|
|
.Dv SX_SLOCKED ,
|
|
or
|
|
.Dv SX_XLOCKED
|
|
assertion:
|
|
.Bl -tag -width ".Dv SX_NOTRECURSED"
|
|
.It Dv SX_RECURSED
|
|
Assert that the current thread has a recursed lock on
|
|
.Fa sx .
|
|
.It Dv SX_NOTRECURSED
|
|
Assert that the current thread does not have a recursed lock on
|
|
.Fa sx .
|
|
.El
|
|
.Pp
|
|
.Fn sx_xlocked
|
|
will return non-zero if the current thread holds the exclusive lock;
|
|
otherwise, it will return zero.
|
|
.Pp
|
|
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
|
|
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
|
|
but with an additional argument,
|
|
.Fa name ,
|
|
that is used in generating unique variable names for the related
|
|
structures associated with the lock and the sysinit routine.
|
|
.Pp
|
|
A thread may not hold both a shared lock and an exclusive lock on the same
|
|
lock simultaneously;
|
|
attempting to do so will result in deadlock.
|
|
.Sh CONTEXT
|
|
A thread may hold a shared or exclusive lock on an
|
|
.Nm
|
|
lock while sleeping.
|
|
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.
|
|
.Sh SEE ALSO
|
|
.Xr locking 9 ,
|
|
.Xr lock 9 ,
|
|
.Xr mutex 9 ,
|
|
.Xr panic 9 ,
|
|
.Xr rwlock 9 ,
|
|
.Xr sema 9
|
|
.Sh BUGS
|
|
Currently there is no way to assert that a lock is not held.
|
|
This is not possible in the
|
|
.No non- Ns Dv WITNESS
|
|
case for asserting that this thread
|
|
does not hold a shared lock.
|
|
In the
|
|
.No non- Ns Dv WITNESS
|
|
case, the
|
|
.Dv SX_LOCKED
|
|
and
|
|
.Dv SX_SLOCKED
|
|
assertions merely check that some thread holds a shared lock.
|
|
They do not ensure that the current thread holds a shared lock.
|