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140 lines
4.3 KiB
140 lines
4.3 KiB
.\"
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.\" Copyright 2013 Samy Al Bahra.
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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 REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER 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
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.\" SUCH DAMAGE.
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.\"
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.\"
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.Dd July 26, 2013.
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.Dt ck_sequence 3
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.Sh NAME
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.Nm ck_sequence
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.Nd sequence locks
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.Sh LIBRARY
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Concurrency Kit (libck, \-lck)
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.Sh SYNOPSIS
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.In ck_sequence.h
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.Pp
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.Dv ck_sequence_t seqlock = CK_SEQUENCE_INITIALIZER;
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.Pp
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.Ft void
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.Fn ck_sequence_init "ck_sequence_t *sq"
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.Ft unsigned int
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.Fn ck_sequence_read_begin "ck_sequence_t *sq"
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.Ft bool
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.Fn ck_sequence_read_retry "ck_sequence_t *sq" "unsigned int version"
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.Ft void
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.Fn ck_sequence_write_begin "ck_sequence_t *sq"
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.Ft void
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.Fn ck_sequence_write_end "ck_sequence_t *sq"
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.Sh DESCRIPTION
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It is recommended to use ck_sequence when a small amount of data that cannot be
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accessed atomically has to be synchronized with readers in a fashion that does
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not block any writer. Readers are able to execute their read-side critical
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sections without any atomic operations. A ck_sequence_t must be initialized
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before use. It may be initialized using either a static initializer
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(CK_SEQUENCE_INITIALIZER) or using
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.Fn ck_sequence_init .
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Before readers attempt to
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read data that may be concurrently modified they must first save the return
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value of
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.Fn ck_sequence_read_begin .
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While or after a reader has completed copying
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the data associated with a ck_sequence_t it must pass the earlier return value
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of
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.Fn ck_sequence_read_begin
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to
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.Fn ck_sequence_read_retry. If
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.Fn ck_sequence_read_retry
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returns true then the copy of data may be inconsistent and the read process
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must be retried. Writers must rely on their own synchronization primitives.
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Once a writer has entered its respective critical section, it must call
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.Fn ck_sequence_write_begin
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to signal intent to update the data protected
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by the ck_sequence_t. Before the writer leaves its critical section it must
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execute
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.Fn ck_sequence_write_end
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to indicate that the updates have left respective objects in a consistent state.
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.Sh EXAMPLE
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.Bd -literal -offset indent
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#include <ck_sequence.h>
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#include <stdlib.h>
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static struct example {
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int a;
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int b;
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int c;
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} global;
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static ck_sequence_t seqlock = CK_SEQUENCE_INITIALIZER;
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void
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reader(void)
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{
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struct example copy;
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unsigned int version;
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/*
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* Attempt a read of the data structure. If the structure
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* has been modified between ck_sequence_read_begin and
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* ck_sequence_read_retry then attempt another read since
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* the data may be in an inconsistent state.
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*/
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do {
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version = ck_sequence_read_begin(&seqlock);
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copy = global;
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} while (ck_sequence_read_retry(&seqlock, version));
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/*
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* The previous may also be expressed using CK_SEQUENCE_READ.
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* Generally recommend to only use ck_sequence_read_retry
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* if you would like to detect a conflicting write at some
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* higher granularity.
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*/
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CK_SEQUENCE_READ(&seqlock, version) {
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copy = global;
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}
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return;
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}
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void
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writer(void)
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{
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for (;;) {
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ck_sequence_write_begin(&seqlock);
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global.a = rand();
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global.b = global.a + global.b;
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global.c = global.b + global.c;
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ck_sequence_write_end(&seqlock);
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}
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return;
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}
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.Ed
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.Sh SEE ALSO
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.Xr ck_bytelock 3 ,
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.Xr ck_rwlock 3
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.Pp
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Additional information available at http://concurrencykit.org/
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