157 lines
4.2 KiB
C
157 lines
4.2 KiB
C
/*-
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* Copyright (c) 2014 Mateusz Guzik <mjg@FreeBSD.org>
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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 AUTHOR 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 AUTHOR 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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* $FreeBSD$
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*/
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#ifndef _SYS_SEQ_H_
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#define _SYS_SEQ_H_
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#ifdef _KERNEL
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#include <sys/systm.h>
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#endif
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#include <sys/types.h>
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/*
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* seq_t may be included in structs visible to userspace
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*/
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typedef uint32_t seq_t;
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#ifdef _KERNEL
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/*
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* seq allows readers and writers to work with a consistent snapshot. Modifying
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* operations must be enclosed within a transaction delineated by
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* seq_write_beg/seq_write_end. The trick works by having the writer increment
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* the sequence number twice, at the beginning and end of the transaction.
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* The reader detects that the sequence number has not changed between its start
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* and end, and that the sequence number is even, to validate consistency.
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*
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* Some fencing (both hard fencing and compiler barriers) may be needed,
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* depending on the cpu. Modern AMD cpus provide strong enough guarantees to not
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* require any fencing by the reader or writer.
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*
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* Example usage:
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*
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* writers:
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* lock_exclusive(&obj->lock);
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* seq_write_begin(&obj->seq);
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* obj->var1 = ...;
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* obj->var2 = ...;
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* seq_write_end(&obj->seq);
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* unlock_exclusive(&obj->lock);
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*
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* readers:
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* int var1, var2;
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* seq_t seq;
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*
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* for (;;) {
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* seq = seq_read(&obj->seq);
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* var1 = obj->var1;
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* var2 = obj->var2;
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* if (seq_consistent(&obj->seq, seq))
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* break;
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* }
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* .....
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*
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* Writers may not block or sleep in any way.
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*
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* There are 2 minor caveats in this implementation:
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*
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* 1. There is no guarantee of progress. That is, a large number of writers can
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* interfere with the execution of the readers and cause the code to live-lock
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* in a loop trying to acquire a consistent snapshot.
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*
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* 2. If the reader loops long enough, the counter may overflow and eventually
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* wrap back to its initial value, fooling the reader into accepting the
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* snapshot. Given that this needs 4 billion transactional writes across a
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* single contended reader, it is unlikely to ever happen.
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*/
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/* A hack to get MPASS macro */
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#include <sys/lock.h>
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#include <machine/cpu.h>
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static __inline bool
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seq_in_modify(seq_t seqp)
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{
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return (seqp & 1);
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}
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static __inline void
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seq_write_begin(seq_t *seqp)
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{
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critical_enter();
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MPASS(!seq_in_modify(*seqp));
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*seqp += 1;
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atomic_thread_fence_rel();
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}
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static __inline void
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seq_write_end(seq_t *seqp)
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{
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atomic_store_rel_int(seqp, *seqp + 1);
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MPASS(!seq_in_modify(*seqp));
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critical_exit();
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}
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static __inline seq_t
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seq_read(const seq_t *seqp)
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{
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seq_t ret;
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for (;;) {
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ret = atomic_load_acq_int(__DECONST(seq_t *, seqp));
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if (seq_in_modify(ret)) {
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cpu_spinwait();
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continue;
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}
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break;
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}
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return (ret);
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}
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static __inline seq_t
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seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
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{
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return (*seqp == oldseq);
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}
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static __inline seq_t
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seq_consistent(const seq_t *seqp, seq_t oldseq)
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{
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atomic_thread_fence_acq();
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return (seq_consistent_nomb(seqp, oldseq));
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}
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#endif /* _KERNEL */
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#endif /* _SYS_SEQ_H_ */
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