linux-kernel - [PATCH 4.17 176/336] arm64: cmpwait: Clear event register before arming exclusive monitor

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Date:   Wed,  1 Aug 2018 18:48:31 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: [PATCH 4.17 176/336] arm64: cmpwait: Clear event register before arming exclusive monitor

4.17-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Will Deacon <will.deacon@....com>

[ Upstream commit 1cfc63b5ae60fe7e01773f38132f98d8b13a99a0 ]

When waiting for a cacheline to change state in cmpwait, we may immediately
wake-up the first time around the outer loop if the event register was
already set (for example, because of the event stream).

Avoid these spurious wakeups by explicitly clearing the event register
before loading the cacheline and setting the exclusive monitor.

Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/arm64/include/asm/cmpxchg.h |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/arm64/include/asm/cmpxchg.h
+++ b/arch/arm64/include/asm/cmpxchg.h
@@ -204,7 +204,9 @@ static inline void __cmpwait_case_##name
 	unsigned long tmp;						\
 									\
 	asm volatile(							\
-	"	ldxr" #sz "\t%" #w "[tmp], %[v]\n"		\
+	"	sevl\n"							\
+	"	wfe\n"							\
+	"	ldxr" #sz "\t%" #w "[tmp], %[v]\n"			\
 	"	eor	%" #w "[tmp], %" #w "[tmp], %" #w "[val]\n"	\
 	"	cbnz	%" #w "[tmp], 1f\n"				\
 	"	wfe\n"							\