linux-kernel - [PATCH RESEND 2/2] ucount: Use atomic_long_try_cmpxchg() in atomic_long_inc

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Message-ID: <20250721174227.28002-2-ubizjak@gmail.com>
Date: Mon, 21 Jul 2025 19:41:44 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Alexey Gladkov <legion@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	MengEn Sun <mengensun@...cent.com>,
	Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH RESEND 2/2] ucount: Use atomic_long_try_cmpxchg() in atomic_long_inc_below()

Use atomic_long_try_cmpxchgi() instead of
atomic_long_cmpxchg (*ptr, old, new) == old in atomic_long_inc_belowi().
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.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Reviewed-by: Alexey Gladkov <legion@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Alexey Gladkov <legion@...nel.org>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: MengEn Sun <mengensun@...cent.com>
Cc: "Thomas Weißschuh" <linux@...ssschuh.net>
---
 kernel/ucount.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index f629db485a07..586af49fc03e 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,
-- 
2.50.1