FAILED: patch "[PATCH] sched/psi: Shut down rtpoll_timer in psi_cgroup_free()" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 5457025fa8ca3c0d2732109513de839e3e797190
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026081706-verdict-hangout-e22e@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5457025fa8ca3c0d2732109513de839e3e797190 Mon Sep 17 00:00:00 2001
From: Tejun Heo <[email protected]>
Date: Sun, 12 Jul 2026 07:23:55 -1000
Subject: [PATCH] sched/psi: Shut down rtpoll_timer in psi_cgroup_free()

psi_schedule_rtpoll_work() is called locklessly from the scheduler hotpath
and can race psi_trigger_destroy() taking down the last rtpoll trigger under
rtpoll_trigger_lock:

  psi_schedule_rtpoll_work()        psi_trigger_destroy()

  rcu_read_lock();
  task = rcu_dereference(rtpoll_task);
                                    rcu_assign_pointer(rtpoll_task, NULL);
                                    timer_delete(&rtpoll_timer);
  mod_timer(&rtpoll_timer, ...);
  rcu_read_unlock();
                                    synchronize_rcu();
                                    kthread_stop(task_to_destroy);

The group can then be freed with the re-armed timer still pending, and
poll_timer_fn() runs on freed memory.

461daba06bdc ("psi: eliminate kthread_worker from psi trigger scheduling
mechanism") deleted the timer synchronously after the synchronize_rcu(),
which prevented this but raced trigger creation instead: the deletion could
cancel the timer that a new trigger set armed during the grace period and,
as creation also reinitialized the timer at the time, corrupt it.
8f91efd870ea ("psi: Fix race between psi_trigger_create/destroy") moved the
initialization into group_init() and the deletion into the locked section,
trading the creation races for the window above.

Neither placement in the destruction path works. A pending timer firing
while the group is alive is harmless though. poll_timer_fn() just wakes the
rtpoll waitqueue and doesn't re-arm itself. Bind the timer to the group's
lifetime instead and shut it down in psi_cgroup_free(). Nothing can arm it
by then. timer_shutdown_sync() because the timer is never armed again.

Fixes: 8f91efd870ea ("psi: Fix race between psi_trigger_create/destroy")
Cc: [email protected] # v5.10+
Reported-by: Sashiko AI <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Tejun Heo <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Tested-by: Matt Fleming <[email protected]>
Acked-by: Suren Baghdasaryan <[email protected]>

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 565ec7b80743..e2e825dcd088 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1134,6 +1134,12 @@ void psi_cgroup_free(struct cgroup *cgroup)
 		return;
 
 	cancel_delayed_work_sync(&cgroup->psi->avgs_work);
+	/*
+	 * A psi_schedule_rtpoll_work() call racing the last trigger's
+	 * destruction may have re-armed the timer after psi_trigger_destroy()
+	 * deleted it. Spurious firing while the group is alive is harmless.
+	 */
+	timer_shutdown_sync(&cgroup->psi->rtpoll_timer);
 	free_percpu(cgroup->psi->pcpu);
 	/* All triggers must be removed by now */
 	WARN_ONCE(cgroup->psi->rtpoll_states, "psi: trigger leak\n");