locking/atomics: Use atomic_try_cmpxchg_release() to micro-optimize rcuref_put_slowpath()

Use atomic_try_cmpxchg() instead of atomic_cmpxchg(*ptr, old, new) == old
in rcuref_put_slowpath(). On x86 the CMPXCHG instruction returns success in the
ZF flag, so this change saves a compare after CMPXCHG.  Additionaly,
the compiler reorders some code blocks to follow likely/unlikely
annotations in the atomic_try_cmpxchg() macro, improving the code from:

  9a:	f0 0f b1 0b          	lock cmpxchg %ecx,(%rbx)
  9e:	83 f8 ff             	cmp    $0xffffffff,%eax
  a1:	74 04                	je     a7 <rcuref_put_slowpath+0x27>
  a3:	31 c0                	xor    %eax,%eax

to:

  9a:	f0 0f b1 0b          	lock cmpxchg %ecx,(%rbx)
  9e:	75 4c                	jne    ec <rcuref_put_slowpath+0x6c>
  a0:	b0 01                	mov    $0x1,%al

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ubizjak

lib/rcuref.c

@@ -248,7 +248,7 @@ bool rcuref_put_slowpath(rcuref_t *ref)
 		 * require a retry. If this fails the caller is not
 		 * allowed to deconstruct the object.
 		 */
-		if (atomic_cmpxchg_release(&ref->refcnt, RCUREF_NOREF, RCUREF_DEAD) != RCUREF_NOREF)
+		if (!atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD))
 			return false;
 
 		/*