linux-kernel - [RFC][PATCH 1/5] mips/atomic: Fix cmpxchg64 barriers

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Date:   Wed, 24 Apr 2019 14:36:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     stern@...land.harvard.edu, akiyks@...il.com,
        andrea.parri@...rulasolutions.com, boqun.feng@...il.com,
        dlustig@...dia.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, npiggin@...il.com, paulmck@...ux.ibm.com,
        peterz@...radead.org, will.deacon@....com
Cc:     linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        Paul Burton <paul.burton@...s.com>
Subject: [RFC][PATCH 1/5] mips/atomic: Fix cmpxchg64 barriers

There were no memory barriers on cmpxchg64() _at_all_. Fix this.

Cc: Paul Burton <paul.burton@...s.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/mips/include/asm/cmpxchg.h |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/arch/mips/include/asm/cmpxchg.h
+++ b/arch/mips/include/asm/cmpxchg.h
@@ -290,9 +290,11 @@ static inline unsigned long __cmpxchg64(
 	 * will cause a build error unless cpu_has_64bits is a		\
 	 * compile-time constant 1.					\
 	 */								\
-	if (cpu_has_64bits && kernel_uses_llsc)				\
+	if (cpu_has_64bits && kernel_uses_llsc) {			\
+		smp_mb__before_llsc();					\
 		__res = __cmpxchg64((ptr), __old, __new);		\
-	else								\
+		smp_llsc_mb();						\
+	} else								\
 		__res = __cmpxchg64_unsupported();			\
 									\
 	__res;								\