linux-kernel - [tip: timers/core] posix-timers: Invoke cond_resched() during exit

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Message-ID: <173987072878.10177.11121566897445550577.tip-bot2@tip-bot2>
Date: Tue, 18 Feb 2025 09:25:28 -0000
From: "tip-bot2 for Benjamin Segall" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Ben Segall <bsegall@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject:
 [tip: timers/core] posix-timers: Invoke cond_resched() during exit_itimers()

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     f99c5bb396b8d1424ed229d1ffa6f596e3b9c36b
Gitweb:        https://git.kernel.org/tip/f99c5bb396b8d1424ed229d1ffa6f596e3b9c36b
Author:        Benjamin Segall <bsegall@...gle.com>
AuthorDate:    Fri, 14 Feb 2025 14:12:20 -08:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Tue, 18 Feb 2025 10:12:49 +01:00

posix-timers: Invoke cond_resched() during exit_itimers()

exit_itimers() loops through every timer in the process to delete it.  This
requires taking the system-wide hash_lock for each of these timers, and
contends with other processes trying to create or delete timers.

When a process creates hundreds of thousands of timers, and then exits
while other processes contend with it, this can trigger softlockups on
CONFIG_PREEMPT=n.

Add a cond_resched() invocation into the loop to allow the system to make
progress.

Signed-off-by: Ben Segall <bsegall@...gle.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Link: https://lore.kernel.org/all/xm2634gg2n23.fsf@google.com
---
 kernel/time/posix-timers.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 1b675ae..44ba7db 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -1099,8 +1099,10 @@ void exit_itimers(struct task_struct *tsk)
 	spin_unlock_irq(&tsk->sighand->siglock);
 
 	/* The timers are not longer accessible via tsk::signal */
-	while (!hlist_empty(&timers))
+	while (!hlist_empty(&timers)) {
 		itimer_delete(hlist_entry(timers.first, struct k_itimer, list));
+		cond_resched();
+	}
 
 	/*
 	 * There should be no timers on the ignored list. itimer_delete() has