137 lines
4.5 KiB
Groff
137 lines
4.5 KiB
Groff
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.\"
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.\" Copyright (C) 2002 Garrett Rooney <rooneg@electricjellyfish.net>.
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.\" 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 March 25, 2002
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.Dt MTX_POOL 9
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.Os
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.Sh NAME
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.Nm mtx_pool ,
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.Nm mtx_pool_alloc ,
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.Nm mtx_pool_find ,
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.Nm mtx_pool_lock ,
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.Nm mtx_pool_unlock
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.Nd "mutex pool routines"
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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/mutex.h
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.Ft struct mtx *
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.Fn mtx_pool_alloc "void"
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.Ft struct mtx *
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.Fn mtx_pool_find "void *ptr"
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.Ft void
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.Fn mtx_pool_lock "void *ptr"
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.Ft void
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.Fn mtx_pool_unlock "void *ptr"
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.Sh DESCRIPTION
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Mutex pools are designed to be used as short term leaf mutexes;
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i.e., the last mutex one might acquire before calling
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.Fn msleep .
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They operate using a shared pool of mutexes.
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A mutex is chosen from the pool based on the supplied pointer,
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which may or may not point to anything valid.
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.Pp
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The shared mutex managed by the pool module are standard, non-recursive,
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blockable mutexes, and should only be used in appropriate situations.
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.Pp
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The caller can lock and unlock mutexes returned by the pool routines, but
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since the mutexes are shared, the caller should not attempt to destroy them
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or modify their characteristics.
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While pool mutexes are normally leaf mutexes
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(meaning that one cannot depend on any ordering guarantees
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after obtaining one),
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one can still obtain other mutexes under carefully controlled circumstances.
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Specifically, if one has a private mutex
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(one that was allocated and initialized by the caller),
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one can obtain it after obtaining a pool mutex if ordering issues are
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carefully accounted for.
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In these cases the private mutex winds up being the true leaf mutex.
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.Pp
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Pool mutexes have the following advantages:
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.Bl -enum -offset indent -compact
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.It
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No structural overhead;
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i.e., they can be associated with a structure without adding bloat to it.
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.It
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Mutexes can be obtained for invalid pointers, which is useful when one uses
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mutexes to interlock destructor operations.
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.It
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No initialization or destruction overhead.
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.It
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Can be used with
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.Fn msleep .
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.El
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.Pp
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And the following disadvantages:
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.Bl -enum -offset indent -compact
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.It
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Should generally only be used as leaf mutexes.
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.It
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Pool/pool dependency ordering cannot be guaranteed.
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.It
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Possible L1 cache mastership contention between CPUs.
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.El
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.Pp
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.Fn mtx_pool_alloc
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obtains a shared mutex from the pool.
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This routine uses a simple rover to choose one of the shared mutexes managed
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by the
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.Nm
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subsystem.
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.Pp
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.Fn mtx_pool_find
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returns the shared mutex associated with the specified address.
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This routine will create a hash out of the pointer passed into it
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and will choose a shared mutex based on that hash.
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The pointer does not need to point to anything real.
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.Pp
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.Fn mtx_pool_lock
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and
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.Fn mtx_pool_unlock
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lock and unlock the shared mutex associated with the specified address,
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respectively;
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they are a combination of
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.Fn mtx_pool_find
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and
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.Fn mtx_lock
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and
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.Fn mtx_unlock ,
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respectively.
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Since these routines must first find the mutex to operate on,
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they are not as fast as directly using the pointer (mutex) returned by
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a previous invocation of
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.Fn mtx_pool_find .
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.Pp
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.Sh SEE ALSO
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.Xr mutex 9 ,
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.Xr msleep 9
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.Sh HISTORY
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These routines first appeared in
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.Fx 5.0 .
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