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/20250721174610.28361-2-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Alexey Gladkov <legion@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Alexey Gladkov <legion@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: MengEn Sun <mengensun@tencent.com>
Cc: "Thomas Weißschuh" <linux@weissschuh.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/ucount.c b/kernel/ucount.c
index f629db485a07972a59ab770fe862319d9b382038..586af49fc03e42be072b2c8c6762dd66c37bc927 100644 (file)
--- a/kernel/ucount.c
+++ b/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,