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fix up r353340, don't assume that fcmpset has strong semantics

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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
This commit is contained in:
avg 2019-10-11 17:01:02 +00:00
parent a850973810
commit 238787c74e
1 changed files with 23 additions and 0 deletions
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23
sys/cddl/compat/opensolaris/sys/atomic.h
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@ -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 delta)
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