arm64: spinlock: Fix theoretical trylock() A-B-A with LSE atomics

[ 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@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
index 95ad7102b63cec8f844e211e10ba5987ea812c62..82375b896be56532cf3fdd3c619338d226cb35f1 100644 (file)
--- a/arch/arm64/include/asm/spinlock.h
+++ b/arch/arm64/include/asm/spinlock.h
@@ -89,8 +89,8 @@ 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)