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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--FJ766Kk/2p6HVARF Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 --FJ766Kk/2p6HVARF Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQIcBAEBAgAGBQJT76jOAAoJEJDCuSvBvK1BrQ8P/0/VURTb8c2uvicU5NNRyJdJ qFNT+pcJkWDXJ0NE02Tm8yLIqKkUcyjvOfYKz8PriXpyfA4odr+PT4SVszgNOJOV 2awtMTkZB+ci3/T/GhVm2IoqD+ulCw/cpzbpMg9rzizF7haWM8G4ie1YY59Ive6x hmFLtQDoSpq5XeNMPUsrA5T+KCEdG4WpC4O749tZfGcstPr8ajsG8u7+gLoC+9rt VMhCzRdHGyvJW+U2yO/iNLR1RepCmHMeFt+Euh9uluRVqD/LN2QO2Z9ZdA6fHYXe fhxSf2gyRoG93I4nvF7ILZ690cwoD/Y2UajTzf6D7j9VddqTPr41ka+u4HgFnQ96 W2p18Cd6SWKJbKwVY39QLApXL9iOUejP8BY1VCVc8wKk4jVqX4/+FXB+I0+B9UGM 0aUfa+GgQNQ4ZKyE+v8b46tQdftV3ebygzO+j9/fZqFkBL+llz+vVCFfLSpTzCWJ mz7v/bNmmObdHIiTTRtsiPIG6qoQTIPIENBhsPuV5Nluc3d4+QMyaD9wrMjhj5Jp oRjLBeRRJtTpdyjpeFDx1KjpoYgLzpq0e6jYGat61Sln+SJ0HhaCFCvFR4st+uMU xVcPMIwat6WxHySu8CJiv9fDPmwUpRh/bIc3AR/sOU/5rLGIPjoqIK99GRqPp8Qr ubqYG5EjczfEmPrioXsJ =/R2K -----END PGP SIGNATURE----- --FJ766Kk/2p6HVARF--
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