[PATCH 4/5] refcount: Use atomic_try_cmpxchg()

[Peter Zijlstra]

Generates better code (GCC-6.2.1):

   text    data     bss     dec     hex filename
   1576       7       0    1583     62f defconfig-build/lib/refcount.o.pre
   1488       7       0    1495     5d7 defconfig-build/lib/refcount.o.post

Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 lib/refcount.c |   47 +++++++++++++++--------------------------------
 1 file changed, 15 insertions(+), 32 deletions(-)

--- a/lib/refcount.c
+++ b/lib/refcount.c
@@ -57,9 +57,9 @@
  */
 bool refcount_add_not_zero(unsigned int i, refcount_t *r)
 {
-       unsigned int old, new, val = atomic_read(&r->refs);
+       unsigned int new, val = atomic_read(&r->refs);
 
-       for (;;) {
+       do {
                if (!val)
                        return false;
 
@@ -69,12 +69,8 @@ bool refcount_add_not_zero(unsigned int
                new = val + i;
                if (new < val)
                        new = UINT_MAX;
-               old = atomic_cmpxchg_relaxed(&r->refs, val, new);
-               if (old == val)
-                       break;
 
-               val = old;
-       }
+       } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
 
        WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
 
@@ -118,9 +114,9 @@ EXPORT_SYMBOL_GPL(refcount_add);
  */
 bool refcount_inc_not_zero(refcount_t *r)
 {
-       unsigned int old, new, val = atomic_read(&r->refs);
+       unsigned int new, val = atomic_read(&r->refs);
 
-       for (;;) {
+       do {
                new = val + 1;
 
                if (!val)
@@ -129,12 +125,7 @@ bool refcount_inc_not_zero(refcount_t *r
                if (unlikely(!new))
                        return true;
 
-               old = atomic_cmpxchg_relaxed(&r->refs, val, new);
-               if (old == val)
-                       break;
-
-               val = old;
-       }
+       } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
 
        WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
 
@@ -182,9 +173,9 @@ EXPORT_SYMBOL_GPL(refcount_inc);
  */
 bool refcount_sub_and_test(unsigned int i, refcount_t *r)
 {
-       unsigned int old, new, val = atomic_read(&r->refs);
+       unsigned int new, val = atomic_read(&r->refs);
 
-       for (;;) {
+       do {
                if (unlikely(val == UINT_MAX))
                        return false;
 
@@ -194,12 +185,7 @@ bool refcount_sub_and_test(unsigned int
                        return false;
                }
 
-               old = atomic_cmpxchg_release(&r->refs, val, new);
-               if (old == val)
-                       break;
-
-               val = old;
-       }
+       } while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
 
        return !new;
 }
@@ -258,7 +244,9 @@ EXPORT_SYMBOL_GPL(refcount_dec);
  */
 bool refcount_dec_if_one(refcount_t *r)
 {
-       return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;
+       int val = 1;
+
+       return atomic_try_cmpxchg_release(&r->refs, &val, 0);
 }
 EXPORT_SYMBOL_GPL(refcount_dec_if_one);
 
@@ -275,9 +263,9 @@ EXPORT_SYMBOL_GPL(refcount_dec_if_one);
  */
 bool refcount_dec_not_one(refcount_t *r)
 {
-       unsigned int old, new, val = atomic_read(&r->refs);
+       unsigned int new, val = atomic_read(&r->refs);
 
-       for (;;) {
+       do {
                if (unlikely(val == UINT_MAX))
                        return true;
 
@@ -290,12 +278,7 @@ bool refcount_dec_not_one(refcount_t *r)
                        return true;
                }
 
-               old = atomic_cmpxchg_release(&r->refs, val, new);
-               if (old == val)
-                       break;
-
-               val = old;
-       }
+       } while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
 
        return true;
 }