Date: Sat, 16 Aug 2014 21:54:06 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Mateusz Guzik <mjguzik@gmail.com> Cc: Johan Schuijt <johan@transip.nl>, freebsd-arch@freebsd.org Subject: Re: [PATCH 1/2] Implement simple sequence counters with memory barriers. Message-ID: <20140816185406.GD2737@kib.kiev.ua> In-Reply-To: <20140816093811.GX2737@kib.kiev.ua> References: <1408064112-573-1-git-send-email-mjguzik@gmail.com> <1408064112-573-2-git-send-email-mjguzik@gmail.com> <20140816093811.GX2737@kib.kiev.ua>
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On Sat, Aug 16, 2014 at 12:38:11PM +0300, Konstantin Belousov wrote:
> 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
> >=20
> > 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 distribut=
ion.
> > + *
> > + * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' =
AND
> > + * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, T=
HE
> > + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR =
PURPOSE
> > + * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LI=
ABLE
> > + * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQ=
UENTIAL
> > + * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE G=
OODS
> > + * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTIO=
N)
> > + * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,=
STRICT
> > + * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN A=
NY 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 =3D seq_read(&gobj->seq);
> > + * lobj =3D gobj;
> > + * if (seq_consistent(&gobj->seq, seq))
> > + * break;
> > + * cpu_spinwait();
> > + * }
> > + * foo(lobj);
> > + */ =09
> > +
> > +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);
Alan Cox rightfully pointed out that better expression is
v =3D *seqp + 1; =
=20
atomic_store_rel_int(seqp, v);
which also takes care of TSO on x86.
> Same note for all other linux-style barriers. In fact, on x86
> wmb() is sfence and it serves no useful purpose in seq_write*.
>=20
> 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 ?
Still, merging it to the guts of lock implementation is right
approach, IMO.
>=20
> > +}
> > +
> > +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 =3D 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 =3D=3D 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_ */
> > --=20
> > 2.0.2
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