Author: kib
Date: Wed Jul 8 18:37:08 2015
New Revision: 285285
URL: https://svnweb.freebsd.org/changeset/base/285285
Log:
Use atomic_fence_fence_rel() to ensure ordering in the
seq_write_begin(), instead of the load_rmb/rbm_load functions. The
update does not need to be atomic due to the write lock owned.
Similarly, in seq_write_end(), update of *seqp needs not be atomic.
Only store must be atomic with release.
For seq_read(), the natural operation is the load acquire of the
sequence value, express this directly with atomic_load_acq_int()
instead of using custom partial fence implementation
atomic_load_rmb_int().
In seq_consistent, use atomic_thread_fence_acq() which provides the
desired semantic of ordering reads before fence before the re-reading
of *seqp, instead of custom atomic_rmb_load_int().
Reviewed by: alc, bde
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
Modified:
head/sys/sys/seq.h
Modified: head/sys/sys/seq.h
==============================================================================
--- head/sys/sys/seq.h Wed Jul 8 18:36:37 2015 (r285284)
+++ head/sys/sys/seq.h Wed Jul 8 18:37:08 2015 (r285285)
@@ -69,35 +69,6 @@ typedef uint32_t seq_t;
#include <machine/cpu.h>
-/*
- * Stuff below is going away when we gain suitable memory barriers.
- *
- * atomic_load_acq_int at least on amd64 provides a full memory barrier,
- * in a way which affects performance.
- *
- * Hack below covers all architectures and avoids most of the penalty at least
- * on amd64 but still has unnecessary cost.
- */
-static __inline int
-atomic_load_rmb_int(volatile const u_int *p)
-{
- volatile u_int v;
-
- v = *p;
- atomic_load_acq_int(&v);
- return (v);
-}
-
-static __inline int
-atomic_rmb_load_int(volatile const u_int *p)
-{
- volatile u_int v = 0;
-
- atomic_load_acq_int(&v);
- v = *p;
- return (v);
-}
-
static __inline bool
seq_in_modify(seq_t seqp)
{
@@ -110,14 +81,15 @@ seq_write_begin(seq_t *seqp)
{
MPASS(!seq_in_modify(*seqp));
- atomic_add_acq_int(seqp, 1);
+ *seqp += 1;
+ atomic_thread_fence_rel();
}
static __inline void
seq_write_end(seq_t *seqp)
{
- atomic_add_rel_int(seqp, 1);
+ atomic_store_rel_int(seqp, *seqp + 1);
MPASS(!seq_in_modify(*seqp));
}
@@ -127,7 +99,7 @@ seq_read(const seq_t *seqp)
seq_t ret;
for (;;) {
- ret = atomic_load_rmb_int(seqp);
+ ret = atomic_load_acq_int(__DECONST(seq_t *, seqp));
if (seq_in_modify(ret)) {
cpu_spinwait();
continue;
@@ -142,7 +114,8 @@ static __inline seq_t
seq_consistent(const seq_t *seqp, seq_t oldseq)
{
- return (atomic_rmb_load_int(seqp) == oldseq);
+ atomic_thread_fence_acq();
+ return (*seqp == oldseq);
}
static __inline seq_t
_______________________________________________
svn-src-all@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/svn-src-all
To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
