linux-kernel - [PATCH 1/2] posix-timers: Make next_posix_timer_id an atomic

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Message-ID: <20250214135911.2037402-2-edumazet@google.com>
Date: Fri, 14 Feb 2025 13:59:10 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel <linux-kernel@...r.kernel.org>, Benjamin Segall <bsegall@...gle.com>, 
	Eric Dumazet <eric.dumazet@...il.com>, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH 1/2] posix-timers: Make next_posix_timer_id an atomic_t

Instead of relying on a global and shared hash_lock
to protect sig->next_posix_timer_id, make it atomic.

This allows the following patch to use RCU.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/linux/sched/signal.h | 2 +-
 kernel/time/posix-timers.c   | 9 +++++----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index d5d03d919df8..72649d7fce2a 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -136,7 +136,7 @@ struct signal_struct {
 #ifdef CONFIG_POSIX_TIMERS
 
 	/* POSIX.1b Interval Timers */
-	unsigned int		next_posix_timer_id;
+	atomic_t		next_posix_timer_id;
 	struct hlist_head	posix_timers;
 	struct hlist_head	ignored_posix_timers;
 
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 1b675aee99a9..204a351a2fd3 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -105,13 +105,14 @@ static int posix_timer_add(struct k_itimer *timer)
 	 * a plan to handle the resulting CRIU regression gracefully.
 	 */
 	for (cnt = 0; cnt <= INT_MAX; cnt++) {
-		spin_lock(&hash_lock);
-		id = sig->next_posix_timer_id;
+		id = atomic_inc_return(&sig->next_posix_timer_id) - 1;
 
-		/* Write the next ID back. Clamp it to the positive space */
-		sig->next_posix_timer_id = (id + 1) & INT_MAX;
+		/* Clamp id to the positive space */
+		id &= INT_MAX;
 
 		head = &posix_timers_hashtable[hash(sig, id)];
+
+		spin_lock(&hash_lock);
 		if (!__posix_timers_find(head, sig, id)) {
 			hlist_add_head_rcu(&timer->t_hash, head);
 			spin_unlock(&hash_lock);
-- 
2.48.1.601.g30ceb7b040-goog