187 lines
5.2 KiB
Groff
187 lines
5.2 KiB
Groff
.\" $NetBSD: pserialize.9,v 1.13 2017/07/03 21:28:48 wiz Exp $
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.\"
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.\" Copyright (c) 2011 The NetBSD Foundation, Inc.
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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, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, 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 NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd January 26, 2016
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.Dt PSERIALIZE 9
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.Os
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.Sh NAME
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.Nm pserialize
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.Nd passive serialization mechanism
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.Sh SYNOPSIS
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.In sys/pserialize.h
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.Ft pserialize_t
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.Fn pserialize_create "void"
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.Ft void
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.Fn pserialize_destroy "pserialize_t psz"
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.Ft int
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.Fn pserialize_read_enter "void"
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.Ft void
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.Fn pserialize_read_exit "int s"
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.Ft void
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.Fn pserialize_perform "pserialize_t psz"
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.\" -----
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.Sh DESCRIPTION
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Passive serialization is a reader / writer synchronisation mechanism
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designed for lock-less read operations.
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The read operations may happen from software interrupt at
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.Dv IPL_SOFTCLOCK .
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.Sh FUNCTIONS
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.Bl -tag -width compact
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.It Fn pserialize_create
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Allocate a new synchronisation object.
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.It Fn pserialize_destroy
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Destroy the synchronisation object.
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No synchronisation activity should happen at this point.
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.It Fn pserialize_read_enter
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Enter the critical path of the reader side.
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Returns an IPL value, which must be passed to
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.Xr pserialize_read_exit 9 .
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Protected code path is not allowed to block.
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.It Fn pserialize_read_exit
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Exit the critical path of the reader side.
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Takes the IPL value returned by
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.Xr pserialize_read_enter 9 .
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.It Fn pserialize_perform
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Perform the passive serialization on the writer side.
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Passing of this function ensures that no readers are in action.
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Writers must be additionally serialized with a separate mechanism,
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e.g.
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.Xr mutex 9 .
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Operation blocks and it may only be performed from thread context.
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.El
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.\" -----
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.Sh EXAMPLES
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Given a global database of frotz records:
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.Bd -literal
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struct frotz {
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...
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struct frotz *f_next;
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};
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static struct {
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kmutex_t lock;
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pserialize_t psz;
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struct frotz *first;
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} frobbotzim __cacheline_aligned;
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.Ed
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.Pp
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Create a frotz and publish it, as a writer:
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.Bd -literal
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struct frotz *f = pool_get(&frotz_pool, PR_WAITOK);
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/* Initialize f. */
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...
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mutex_enter(&frobbotzim.lock);
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f->f_next = frobbotzim.first;
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/*
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* Publish the contents of f->f_next before we publish the
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* pointer to f in frobbotzim.first.
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*/
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membar_producer();
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frobbotzim.first = f;
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mutex_exit(&frobbotzim.lock);
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.Ed
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.Pp
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Find a frotz, as a reader:
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.Bd -literal
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struct frotz *f;
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int error = ENOENT;
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int s;
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s = pserialize_read_enter();
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for (f = frobbotzim.first; f != NULL; f = f->f_next) {
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/* Fetch f before we fetch anything f points to. */
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membar_datadep_consumer();
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if (f->f_... == key) {
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/*
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* Grab whatever part of the frotz we need.
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* Note that we can't use the frotz after
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* pserialize_read_exit, without a stronger
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* kind of reference, say a reference count
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* managed by atomic_ops(3).
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*/
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*resultp = f->f_...;
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error = 0;
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break;
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}
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}
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pserialize_read_exit(s);
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return error;
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.Ed
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.Pp
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Remove a frotz, as a writer, and free it once there are no more
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readers:
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.Bd -literal
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struct frotz **fp, *f;
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mutex_enter(&frobbotzim.lock);
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for (fp = &frobbotzim.first; (f = *fp) != NULL; fp = &f->f_next) {
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if (f->f_... == key) {
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/*
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* Unhook it from the list. Readers may still
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* be traversing the list at this point, so
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* the next pointer must remain valid and
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* memory must remain allocated.
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*/
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*fp = f->f_next;
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break;
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}
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}
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/*
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* Wait for all existing readers to complete. New readers will
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* not see f because the list no longer points to it.
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*/
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pserialize_perform(frobbotzim.psz);
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/* Now nobody else can be touching f, so it is safe to free. */
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mutex_exit(&frobbotzim.lock);
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if (f != NULL)
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pool_put(&frotz_pool, f);
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.Ed
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.\" -----
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.Sh CODE REFERENCES
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The
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.Nm
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is implemented within the file
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.Pa sys/kern/subr_pserialize.c .
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.Sh SEE ALSO
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.Xr membar_ops 3 ,
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.Xr condvar 9 ,
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.Xr mutex 9 ,
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.Xr rwlock 9
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.Rs
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.%A Hennessy, et al.
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.%T "Passive serialization in a multitasking environment"
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.%I US Patent and Trademark Office
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.%D February 28, 1989
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.%N US Patent 4809168
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.Re
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.Sh HISTORY
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Passive serialization mechanism first appeared in
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.Nx 6.0 .
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