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

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Date:   Wed, 13 Jul 2022 17:54:05 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 2/2] 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.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 kernel/ucount.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 974ac1585009..5d980460104e 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -218,16 +218,14 @@ 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,
-- 
2.35.3