linux-kernel - [PATCH 2/4] ucount: Replace get_ucounts_or_wrap() with atomic_inc_not

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Message-ID: <20250203150525.456525-3-bigeasy@linutronix.de>
Date: Mon,  3 Feb 2025 16:05:23 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	MengEn Sun <mengensun@...cent.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	YueHong Wu <yuehongwu@...cent.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 2/4] ucount: Replace get_ucounts_or_wrap() with atomic_inc_not_zero().

get_ucounts_or_wrap() increments the counter and if the counter is
negative then it decrements it again in order to reset the previous increment.
This statement can be replaced with atomic_inc_not_zero() to only
increment the counter if it is not yet 0.
This simplifies the get function because the put (if the get failed) can
be removed. atomic_inc_not_zero() is implement as a cmpxchg() loop which
can be repeated several times if another get/put is performed in parallel. This
will be optimized later.

Increment the reference counter only if not yet dropped to zero.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/ucount.c | 24 ++++++------------------
 1 file changed, 6 insertions(+), 18 deletions(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 86c5f1c0bad90..4aa5011538257 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -146,25 +146,16 @@ static void hlist_add_ucounts(struct ucounts *ucounts)
 	spin_unlock_irq(&ucounts_lock);
 }
 
-static inline bool get_ucounts_or_wrap(struct ucounts *ucounts)
-{
-	/* Returns true on a successful get, false if the count wraps. */
-	return !atomic_add_negative(1, &ucounts->count);
-}
-
 struct ucounts *get_ucounts(struct ucounts *ucounts)
 {
-	if (!get_ucounts_or_wrap(ucounts)) {
-		put_ucounts(ucounts);
-		ucounts = NULL;
-	}
-	return ucounts;
+	if (atomic_inc_not_zero(&ucounts->count))
+		return ucounts;
+	return NULL;
 }
 
 struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 {
 	struct hlist_head *hashent = ucounts_hashentry(ns, uid);
-	bool wrapped;
 	struct ucounts *ucounts, *new = NULL;
 
 	spin_lock_irq(&ucounts_lock);
@@ -189,14 +180,11 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 			return new;
 		}
 	}
-
-	wrapped = !get_ucounts_or_wrap(ucounts);
+	if (!atomic_inc_not_zero(&ucounts->count))
+		ucounts = NULL;
 	spin_unlock_irq(&ucounts_lock);
 	kfree(new);
-	if (wrapped) {
-		put_ucounts(ucounts);
-		return NULL;
-	}
+
 	return ucounts;
 }
 
-- 
2.47.2