Date: Sat, 4 Oct 2014 08:03:53 +0000 (UTC) From: Mateusz Guzik <mjg@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r272503 - head/sys/sys Message-ID: <201410040803.s9483rJH098388@svn.freebsd.org>
next in thread | raw e-mail | index | archive | help
Author: mjg Date: Sat Oct 4 08:03:52 2014 New Revision: 272503 URL: https://svnweb.freebsd.org/changeset/base/272503 Log: Add sequence counters with memory barriers. Current implementation is somewhat simplistic and hackish, will be improved later after possible memory barrier overhaul. Reviewed by: kib MFC after: 3 weeks Added: head/sys/sys/seq.h (contents, props changed) Added: head/sys/sys/seq.h ============================================================================== --- /dev/null 00:00:00 1970 (empty, because file is newly added) +++ head/sys/sys/seq.h Sat Oct 4 08:03:52 2014 (r272503) @@ -0,0 +1,139 @@ +/*- + * 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 + +/* + * 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> + +/* + * This is a temporary hack until memory barriers are cleaned up. + * + * atomic_load_acq_int at least on amd64 provides a full memory barrier, + * in a way which affects perforance. + * + * Hack below covers all architectures and avoids most of the penalty at least + * on amd64. + */ +static __inline int +atomic_load_acq_rmb_int(volatile u_int *p) +{ + volatile u_int v; + + v = *p; + atomic_load_acq_int(&v); + return (v); +} + +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)); + atomic_add_acq_int(seqp, 1); +} + +static __inline void +seq_write_end(seq_t *seqp) +{ + + atomic_add_rel_int(seqp, 1); + MPASS(!seq_in_modify(*seqp)); +} + +static __inline seq_t +seq_read(seq_t *seqp) +{ + seq_t ret; + + for (;;) { + ret = atomic_load_acq_rmb_int(seqp); + if (seq_in_modify(ret)) { + cpu_spinwait(); + continue; + } + break; + } + + return (ret); +} + +static __inline seq_t +seq_consistent(seq_t *seqp, seq_t oldseq) +{ + + return (atomic_load_acq_rmb_int(seqp) == oldseq); +} + +static __inline seq_t +seq_consistent_nomb(seq_t *seqp, seq_t oldseq) +{ + + return (*seqp == oldseq); +} + +#endif /* _KERNEL */ +#endif /* _SYS_SEQ_H_ */
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?201410040803.s9483rJH098388>