ucount: use atomic_long_try_cmpxchg() in atomic_long_inc_below()

Use atomic_long_try_cmpxchg() instead of
atomic_long_cmpxchg (*ptr, old, new) == old in atomic_long_inc_below().
x86 CMPXCHG instruction returns success in ZF flag, so this change saves
a compare after cmpxchg (and related move instruction in front of cmpxchg).

Also, atomic_long_try_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications.

No functional change intended.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Uros Bizjak <[email protected]>
Reviewed-by: Alexey Gladkov <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Alexey Gladkov <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: MengEn Sun <[email protected]>
Cc: "Thomas Weißschuh" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ubizjak

kernel/ucount.c

@@ -201,16 +201,14 @@ void put_ucounts(struct ucounts *ucounts)
 
 static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
 {
-	long c, old;
-	c = atomic_long_read(v);
-	for (;;) {
+	long c = atomic_long_read(v);
+
+	do {
 		if (unlikely(c >= u))
 			return false;
-		old = atomic_long_cmpxchg(v, c, c+1);
-		if (likely(old == c))
-			return true;
-		c = old;
-	}
+	} while (!atomic_long_try_cmpxchg(v, &c, c+1));
+
+	return true;
 }
 
 struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,