From 58b4fba81a2e400a47ddbe7c1dc0a2bc038313b7 Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Mon, 21 Jul 2025 19:45:58 +0200 Subject: [PATCH] ucount: use atomic_long_try_cmpxchg() in atomic_long_inc_below() MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Alexey Gladkov Cc: Sebastian Andrzej Siewior Cc: "Paul E. McKenney" Cc: Alexey Gladkov Cc: Roman Gushchin Cc: MengEn Sun Cc: "Thomas Weißschuh" Signed-off-by: Andrew Morton --- kernel/ucount.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/kernel/ucount.c b/kernel/ucount.c index f629db485a079..586af49fc03e4 100644 --- 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, -- 2.47.2