linux-kernel - [PATCH AUTOSEL for 4.15 156/189] arm64: spinlock: Fix theoretical trylock() A-B-A with LSE atomics

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Date:   Mon, 9 Apr 2018 00:18:53 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.15 156/189] arm64: spinlock: Fix theoretical
 trylock() A-B-A with LSE atomics

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

[ Upstream commit 202fb4ef81e3ec765c23bd1e6746a5c25b797d0e ]

If the spinlock "next" ticket wraps around between the initial LDR
and the cmpxchg in the LSE version of spin_trylock, then we can erroneously
think that we have successfuly acquired the lock because we only check
whether the next ticket return by the cmpxchg is equal to the owner ticket
in our updated lock word.

This patch fixes the issue by performing a full 32-bit check of the lock
word when trying to determine whether or not the CASA instruction updated
memory.

Reported-by: Catalin Marinas <catalin.marinas@....com>
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>
---
 arch/arm64/include/asm/spinlock.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
index fdb827c7832f..ebdae15d665d 100644
--- a/arch/arm64/include/asm/spinlock.h
+++ b/arch/arm64/include/asm/spinlock.h
@@ -87,8 +87,8 @@ static inline int arch_spin_trylock(arch_spinlock_t *lock)
 	"	cbnz	%w1, 1f\n"
 	"	add	%w1, %w0, %3\n"
 	"	casa	%w0, %w1, %2\n"
-	"	and	%w1, %w1, #0xffff\n"
-	"	eor	%w1, %w1, %w0, lsr #16\n"
+	"	sub	%w1, %w1, %3\n"
+	"	eor	%w1, %w1, %w0\n"
 	"1:")
 	: "=&r" (lockval), "=&r" (tmp), "+Q" (*lock)
 	: "I" (1 << TICKET_SHIFT)
-- 
2.15.1