linux-kernel - [PATCH 2/2] posix-timers: Use RCU in posix_timer

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Message-ID: <20250214135911.2037402-3-edumazet@google.com>
Date: Fri, 14 Feb 2025 13:59:11 +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 2/2] posix-timers: Use RCU in posix_timer_add()

If many posix timers are hashed in posix_timers_hashtable,
hash_lock can be held for long durations.

This can be really bad in some cases as Thomas
explained in https://lore.kernel.org/all/87ednpyyeo.ffs@tglx/

We can perform all searches under RCU, then acquire
the lock only when there is a good chance to need it,
and after cpu caches were populated.

I also added a cond_resched() in the possible long loop.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 kernel/time/posix-timers.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 204a351a2fd3..dd2f9016d3dc 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -112,7 +112,19 @@ static int posix_timer_add(struct k_itimer *timer)
 
 		head = &posix_timers_hashtable[hash(sig, id)];
 
+		rcu_read_lock();
+		if (__posix_timers_find(head, sig, id)) {
+			rcu_read_unlock();
+			cond_resched();
+			continue;
+		}
+		rcu_read_unlock();
 		spin_lock(&hash_lock);
+		/*
+		 * We must perform the lookup under hash_lock protection
+		 * because another thread could have used the same id.
+		 * This is very unlikely, but possible.
+		 */
 		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