From 238787c74e737e271f17330fbad900acc35651cc Mon Sep 17 00:00:00 2001
From: avg <avg@FreeBSD.org>
Date: Fri, 11 Oct 2019 17:01:02 +0000
Subject: [PATCH] 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
---
 sys/cddl/compat/opensolaris/sys/atomic.h | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/sys/cddl/compat/opensolaris/sys/atomic.h b/sys/cddl/compat/opensolaris/sys/atomic.h
index de792c293d37..9e30f3da6669 100644
--- a/sys/cddl/compat/opensolaris/sys/atomic.h
+++ b/sys/cddl/compat/opensolaris/sys/atomic.h
@@ -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