ARM: 6211/1: atomic ops: fix register constraints for atomic64_add_unless

The atomic64_add_unless function compares an atomic variable with
a given value and, if they are not equal, adds another given value
to the atomic variable. The function returns zero if the addition
did not occur and non-zero otherwise.

On ARM, the return value is initialised to 1 in C code. Inline assembly
code then performs the atomic64_add_unless operation, setting the
return value to 0 iff the addition does not occur. This means that
when the addition *does* occur, the value of ret must be preserved
across the inline assembly and therefore requires a "+r" constraint
rather than the current one of "=&r".

Thanks to Nicolas Pitre for helping to spot this.

Cc: stable@kernel.org
Reviewed-by: NNicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/atomic.h

@@ -440,7 +440,7 @@ static inline int atomic64_add_unless(atomic64_t *v, u64 a, u64 u)
 "	teq	%2, #0\n"
 "	bne	1b\n"
 "2:"
-	: "=&r" (val), "=&r" (ret), "=&r" (tmp)
+	: "=&r" (val), "+r" (ret), "=&r" (tmp)
 	: "r" (&v->counter), "r" (u), "r" (a)
 	: "cc");