alpha: Improve atomic_add_unless

Use ll/sc loops instead of C loops around cmpxchg.
Update the atomic64_add_unless block comment to match the code.

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>

1 files changed, 37 insertions, 23 deletions

diff --git a/arch/alpha/include/asm/atomic.h b/arch/alpha/include/asm/atomic.h
index c2cbe4fc391c..0dc18fc4d925 100644
--- a/arch/alpha/include/asm/atomic.h
+++ b/arch/alpha/include/asm/atomic.h
@@ -186,17 +186,24 @@ static __inline__ long atomic64_sub_return(long i, atomic64_t * v)
  */
 static __inline__ int __atomic_add_unless(atomic_t *v, int a, int u)
 {
-        int c, old;
-        c = atomic_read(v);
-        for (;;) {
-                if (unlikely(c == (u)))
-                        break;
-                old = atomic_cmpxchg((v), c, c + (a));
-                if (likely(old == c))
-                        break;
-                c = old;
-        }
-        return c;
+        int c, new, old;
+        smp_mb();
+        __asm__ __volatile__(
+        "1:     ldl_l   %[old],%[mem]\n"
+        "       cmpeq   %[old],%[u],%[c]\n"
+        "       addl    %[old],%[a],%[new]\n"
+        "       bne     %[c],2f\n"
+        "       stl_c   %[new],%[mem]\n"
+        "       beq     %[new],3f\n"
+        "2:\n"
+        ".subsection 2\n"
+        "3:     br      1b\n"
+        ".previous"
+        : [old] "=&r"(old), [new] "=&r"(new), [c] "=&r"(c)
+        : [mem] "m"(*v), [a] "rI"(a), [u] "rI"((long)u)
+        : "memory");
+        smp_mb();
+        return old;
 }
 
 
@@ -207,21 +214,28 @@ static __inline__ int __atomic_add_unless(atomic_t *v, int a, int u)
  * @u: ...unless v is equal to u.
  *
  * Atomically adds @a to @v, so long as it was not @u.
- * Returns the old value of @v.
+ * Returns true iff @v was not @u.
  */
 static __inline__ int atomic64_add_unless(atomic64_t *v, long a, long u)
 {
-        long c, old;
-        c = atomic64_read(v);
-        for (;;) {
-                if (unlikely(c == (u)))
-                        break;
-                old = atomic64_cmpxchg((v), c, c + (a));
-                if (likely(old == c))
-                        break;
-                c = old;
-        }
-        return c != (u);
+        long c, tmp;
+        smp_mb();
+        __asm__ __volatile__(
+        "1:     ldq_l   %[tmp],%[mem]\n"
+        "       cmpeq   %[tmp],%[u],%[c]\n"
+        "       addq    %[tmp],%[a],%[tmp]\n"
+        "       bne     %[c],2f\n"
+        "       stq_c   %[tmp],%[mem]\n"
+        "       beq     %[tmp],3f\n"
+        "2:\n"
+        ".subsection 2\n"
+        "3:     br      1b\n"
+        ".previous"
+        : [tmp] "=&r"(tmp), [c] "=&r"(c)
+        : [mem] "m"(*v), [a] "rI"(a), [u] "rI"(u)
+        : "memory");
+        smp_mb();
+        return !c;
 }
 
 #define atomic64_inc_not_zero(v) atomic64_add_unless((v), 1, 0)