[PATCH 1/2] Implement simple sequence counters with memory barriers.

[Konstantin Belousov]

On Fri, Aug 15, 2014 at 02:55:11AM +0200, Mateusz Guzik wrote:
> ---
>  sys/sys/seq.h | 126 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 126 insertions(+)
>  create mode 100644 sys/sys/seq.h
> 
> diff --git a/sys/sys/seq.h b/sys/sys/seq.h
> new file mode 100644
> index 0000000..0971aef
> --- /dev/null
> +++ b/sys/sys/seq.h
> @@ -0,0 +1,126 @@
> +/*-
> + * Copyright (c) 2014 The FreeBSD Project
> + *
> + * 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
> +
> +/*
> + * Typical usage:
> + *
> + * writers:
> + * 	lock_exclusive(&obj->lock);
> + * 	seq_write_begin(&obj->seq);
> + * 	.....
> + * 	seq_write_end(&obj->seq);
> + * 	unlock_exclusive(&obj->unlock);
> + *
> + * readers:
> + * 	obj_t lobj;
> + * 	seq_t seq;
> + *
> + * 	for (;;) {
> + * 		seq = seq_read(&gobj->seq);
> + * 		lobj = gobj;
> + * 		if (seq_consistent(&gobj->seq, seq))
> + * 			break;
> + * 		cpu_spinwait();
> + * 	}
> + * 	foo(lobj);
> + */		
> +
> +typedef uint32_t seq_t;
> +
> +/* A hack to get MPASS macro */
> +#include <sys/systm.h>
> +#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)
> +{
> +
> +	MPASS(!seq_in_modify(*seqp));
> +	(*seqp)++;
> +	wmb();
This probably ought to be written as atomic_add_rel_int(seqp, 1);
Same note for all other linux-style barriers.  In fact, on x86
wmb() is sfence and it serves no useful purpose in seq_write*.

Overall, it feels too alien and linux-ish for my taste.
Since we have sequence bound to some lock anyway, could we introduce
some sort of generation-aware locks variants, which extend existing
locks, and where lock/unlock bump generation number ?

> +}
> +
> +static __inline void
> +seq_write_end(seq_t *seqp)
> +{
> +
> +	wmb();
> +	(*seqp)++;
> +	MPASS(!seq_in_modify(*seqp));
> +}
> +
> +static __inline seq_t
> +seq_read(seq_t *seqp)
> +{
> +	seq_t ret;
> +
> +	for (;;) {
> +		ret = READ_ONCE(*seqp);
> +		if (seq_in_modify(ret)) {
> +			cpu_spinwait();
> +			continue;
> +		}
> +		break;
> +	}
> +
> +	rmb();
> +
> +	return (ret);
> +}
> +
> +static __inline seq_t
> +seq_consistent_nomb(seq_t *seqp, seq_t oldseqp)
> +{
> +
> +	MPASS(!seq_in_modify(oldseqp));
> +	return (*seqp == oldseqp);
> +}
> +
> +static __inline seq_t
> +seq_consistent(seq_t *seqp, seq_t oldseqp)
> +{
> +
> +	rmb();
> +	return (seq_consistent_nomb(seqp, oldseqp));
> +}
> +
> +#endif	/* _KERNEL */
> +#endif	/* _SYS_SEQ_H_ */
> -- 
> 2.0.2
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