arm64: spinlock: retry trylock operation if strex fails on free lock

An exclusive store instruction may fail for reasons other than lock
contention (e.g. a cache eviction during the critical section) so, in
line with other architectures using similar exclusive instructions
(alpha, mips, powerpc), retry the trylock operation if the lock appears
to be free but the strex reported failure.
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>
Reported-by: NTony Thompson <anthony.thompson@arm.com>
Acked-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/include/asm/spinlock.h

@@ -59,9 +59,10 @@ static inline int arch_spin_trylock(arch_spinlock_t *lock)
 	unsigned int tmp;
 
 	asm volatile(
-	"	ldaxr	%w0, %1\n"
+	"2:	ldaxr	%w0, %1\n"
 	"	cbnz	%w0, 1f\n"
 	"	stxr	%w0, %w2, %1\n"
+	"	cbnz	%w0, 2b\n"
 	"1:\n"
 	: "=&r" (tmp), "+Q" (lock->lock)
 	: "r" (1)