/*-
* Copyright (c) 2014 Mateusz Guzik <mjg@FreeBSD.org>
*
* 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, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, 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 AUTHOR AND CONTRIBUTORS ``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 AUTHOR OR CONTRIBUTORS 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.
*
* $FreeBSD$
*/
#ifndef _SYS_SEQ_H_
#define _SYS_SEQ_H_
#ifdef _KERNEL
#include <sys/systm.h>
#endif
#include <sys/types.h>
/*
* seq_t may be included in structs visible to userspace
*/
typedef uint32_t seq_t;
#ifdef _KERNEL
/*
* seq allows readers and writers to work with a consistent snapshot. Modifying
* operations must be enclosed within a transaction delineated by
* seq_write_beg/seq_write_end. The trick works by having the writer increment
* the sequence number twice, at the beginning and end of the transaction.
* The reader detects that the sequence number has not changed between its start
* and end, and that the sequence number is even, to validate consistency.
*
* Some fencing (both hard fencing and compiler barriers) may be needed,
* depending on the cpu. Modern AMD cpus provide strong enough guarantees to not
* require any fencing by the reader or writer.
*
* Example usage:
*
* writers:
* lock_exclusive(&obj->lock);
* seq_write_begin(&obj->seq);
* obj->var1 = ...;
* obj->var2 = ...;
* seq_write_end(&obj->seq);
* unlock_exclusive(&obj->lock);
*
* readers:
* int var1, var2;
* seq_t seq;
*
* for (;;) {
* seq = seq_read(&obj->seq);
* var1 = obj->var1;
* var2 = obj->var2;
* if (seq_consistent(&obj->seq, seq))
* break;
* }
* .....
*
* Writers may not block or sleep in any way.
*
* There are 2 minor caveats in this implementation:
*
* 1. There is no guarantee of progress. That is, a large number of writers can
* interfere with the execution of the readers and cause the code to live-lock
* in a loop trying to acquire a consistent snapshot.
*
* 2. If the reader loops long enough, the counter may overflow and eventually
* wrap back to its initial value, fooling the reader into accepting the
* snapshot. Given that this needs 4 billion transactional writes across a
* single contended reader, it is unlikely to ever happen.
*/
/* A hack to get MPASS macro */
#include <sys/lock.h>
#include <machine/cpu.h>
static __inline bool
seq_in_modify(seq_t seqp)
{
return (seqp & 1);
}
static __inline void
seq_write_begin(seq_t *seqp)
{
critical_enter();
MPASS(!seq_in_modify(*seqp));
*seqp += 1;
atomic_thread_fence_rel();
}
static __inline void
seq_write_end(seq_t *seqp)
{
atomic_store_rel_32(seqp, *seqp + 1);
MPASS(!seq_in_modify(*seqp));
critical_exit();
}
static __inline seq_t
seq_read(const seq_t *seqp)
{
seq_t ret;
for (;;) {
ret = atomic_load_acq_32(__DECONST(seq_t *, seqp));
if (seq_in_modify(ret)) {
cpu_spinwait();
continue;
}
break;
}
return (ret);
}
static __inline seq_t
seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
{
return (*seqp == oldseq);
}
static __inline seq_t
seq_consistent(const seq_t *seqp, seq_t oldseq)
{
atomic_thread_fence_acq();
return (seq_consistent_nomb(seqp, oldseq));
}
#endif /* _KERNEL */
#endif /* _SYS_SEQ_H_ */
