alpha: Implement atomic64_dec_if_positive

Reviewed-and-Tested-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>

arch/alpha/Kconfig

@@ -15,6 +15,7 @@ config ALPHA
 	select ARCH_WANT_OPTIONAL_GPIOLIB
 	select ARCH_WANT_IPC_PARSE_VERSION
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
+	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
 	select GENERIC_SMP_IDLE_THREAD
 	select GENERIC_CMOS_UPDATE
 	select GENERIC_STRNCPY_FROM_USER

arch/alpha/include/asm/atomic.h

@@ -238,6 +238,34 @@ static __inline__ int atomic64_add_unless(atomic64_t *v, long a, long u)
 	return !c;
 }
 
+/*
+ * atomic64_dec_if_positive - decrement by 1 if old value positive
+ * @v: pointer of type atomic_t
+ *
+ * The function returns the old value of *v minus 1, even if
+ * the atomic variable, v, was not decremented.
+ */
+static inline long atomic64_dec_if_positive(atomic64_t *v)
+{
+	long old, tmp;
+	smp_mb();
+	__asm__ __volatile__(
+	"1:	ldq_l	%[old],%[mem]\n"
+	"	subq	%[old],1,%[tmp]\n"
+	"	ble	%[old],2f\n"
+	"	stq_c	%[tmp],%[mem]\n"
+	"	beq	%[tmp],3f\n"
+	"2:\n"
+	".subsection 2\n"
+	"3:	br	1b\n"
+	".previous"
+	: [old] "=&r"(old), [tmp] "=&r"(tmp)
+	: [mem] "m"(*v)
+	: "memory");
+	smp_mb();
+	return old - 1;
+}
+
 #define atomic64_inc_not_zero(v) atomic64_add_unless((v), 1, 0)
 
 #define atomic_add_negative(a, v) (atomic_add_return((a), (v)) < 0)