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>

2 files changed, 29 insertions, 0 deletions

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 837a1f2d8b96..082d9b4b5472 100644
--- a/arch/alpha/Kconfig
+++ b/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
diff --git a/arch/alpha/include/asm/atomic.h b/arch/alpha/include/asm/atomic.h
index 0dc18fc4d925..78b03ef39f6f 100644
--- a/arch/alpha/include/asm/atomic.h
+++ b/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)