00915f067f
Impact on capability races was small: it was possible to get a spurious ENOTCAPABLE (early return), but it was not possible to bypass checks. Tidy up some comments.
157 lines
3.3 KiB
C
157 lines
3.3 KiB
C
/*-
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* Copyright (c) 2014 Mateusz Guzik <mjg@FreeBSD.org>
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*
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* $FreeBSD$
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*/
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#ifndef _SYS_SEQ_H_
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#define _SYS_SEQ_H_
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#ifdef _KERNEL
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#include <sys/systm.h>
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#endif
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#include <sys/types.h>
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/*
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* seq_t may be included in structs visible to userspace
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*/
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typedef uint32_t seq_t;
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#ifdef _KERNEL
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/*
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* Typical usage:
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*
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* writers:
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* lock_exclusive(&obj->lock);
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* seq_write_begin(&obj->seq);
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* .....
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* seq_write_end(&obj->seq);
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* unlock_exclusive(&obj->unlock);
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*
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* readers:
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* obj_t lobj;
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* seq_t seq;
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*
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* for (;;) {
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* seq = seq_read(&gobj->seq);
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* lobj = gobj;
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* if (seq_consistent(&gobj->seq, seq))
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* break;
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* cpu_spinwait();
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* }
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* foo(lobj);
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*/
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/* A hack to get MPASS macro */
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#include <sys/lock.h>
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#include <machine/cpu.h>
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/*
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* Stuff below is going away when we gain suitable memory barriers.
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*
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* atomic_load_acq_int at least on amd64 provides a full memory barrier,
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* in a way which affects performance.
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*
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* Hack below covers all architectures and avoids most of the penalty at least
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* on amd64 but still has unnecessary cost.
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*/
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static __inline int
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atomic_load_rmb_int(volatile u_int *p)
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{
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volatile u_int v;
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v = *p;
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atomic_load_acq_int(&v);
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return (v);
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}
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static __inline int
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atomic_rmb_load_int(volatile u_int *p)
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{
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volatile u_int v = 0;
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atomic_load_acq_int(&v);
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v = *p;
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return (v);
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}
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static __inline bool
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seq_in_modify(seq_t seqp)
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{
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return (seqp & 1);
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}
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static __inline void
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seq_write_begin(seq_t *seqp)
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{
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MPASS(!seq_in_modify(*seqp));
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atomic_add_acq_int(seqp, 1);
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}
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static __inline void
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seq_write_end(seq_t *seqp)
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{
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atomic_add_rel_int(seqp, 1);
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MPASS(!seq_in_modify(*seqp));
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}
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static __inline seq_t
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seq_read(seq_t *seqp)
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{
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seq_t ret;
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for (;;) {
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ret = atomic_load_rmb_int(seqp);
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if (seq_in_modify(ret)) {
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cpu_spinwait();
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continue;
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}
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break;
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}
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return (ret);
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}
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static __inline seq_t
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seq_consistent(seq_t *seqp, seq_t oldseq)
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{
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return (atomic_rmb_load_int(seqp) == oldseq);
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}
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static __inline seq_t
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seq_consistent_nomb(seq_t *seqp, seq_t oldseq)
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{
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return (*seqp == oldseq);
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}
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#endif /* _KERNEL */
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#endif /* _SYS_SEQ_H_ */
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