Author: avg
Date: Fri Oct 11 17:01:02 2019
New Revision: 353442
URL: https://svnweb.freebsd.org/changeset/base/353442
Log:
fix up r353340, don't assume that fcmpset has strong semantics
fcmpset can have two kinds of semantics, weak and strong.
For practical purposes, strong semantics means that if fcmpset fails
then the reported current value is always different from the expected
value. Weak semantics means that the reported current value may be the
same as the expected value even though fcmpset failed. That's a so
called "sporadic" failure.
I originally implemented atomic_cas expecting strong semantics, but many
platforms actually have weak one.
Reported by: pkubaj (not confirmed if same issue)
Discussed with: kib, mjg
MFC after: 19 days
X-MFC with: r353340
Modified:
head/sys/cddl/compat/opensolaris/sys/atomic.h
Modified: head/sys/cddl/compat/opensolaris/sys/atomic.h
==============================================================================
--- head/sys/cddl/compat/opensolaris/sys/atomic.h Fri Oct 11 16:28:46
2019 (r353441)
+++ head/sys/cddl/compat/opensolaris/sys/atomic.h Fri Oct 11 17:01:02
2019 (r353442)
@@ -36,6 +36,11 @@
#define I386_HAVE_ATOMIC64
#endif
+#if defined(__i386__) || defined(__amd64__) || defined(__arm__)
+/* No spurious failures from fcmpset. */
+#define STRONG_FCMPSET
+#endif
+
#if !defined(__LP64__) && !defined(__mips_n32) && \
!defined(ARM_HAVE_ATOMIC64) && !defined(I386_HAVE_ATOMIC64)
extern void atomic_add_64(volatile uint64_t *target, int64_t delta);
@@ -89,7 +94,16 @@ atomic_dec_32_nv(volatile uint32_t *target)
static inline uint32_t
atomic_cas_32(volatile uint32_t *target, uint32_t cmp, uint32_t newval)
{
+#ifdef STRONG_FCMPSET
(void)atomic_fcmpset_32(target, &cmp, newval);
+#else
+ uint32_t expected = cmp;
+
+ do {
+ if (atomic_fcmpset_32(target, &cmp, newval))
+ break;
+ } while (cmp == expected);
+#endif
return (cmp);
}
#endif
@@ -112,7 +126,16 @@ atomic_add_64_nv(volatile uint64_t *target, int64_t de
static inline uint64_t
atomic_cas_64(volatile uint64_t *target, uint64_t cmp, uint64_t newval)
{
+#ifdef STRONG_FCMPSET
(void)atomic_fcmpset_64(target, &cmp, newval);
+#else
+ uint64_t expected = cmp;
+
+ do {
+ if (atomic_fcmpset_64(target, &cmp, newval))
+ break;
+ } while (cmp == expected);
+#endif
return (cmp);
}
#endif
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