From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Sashiko AI <sashiko-bot@kernel.org>,
	Tejun Heo <tj@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Matt Fleming <mfleming@cloudflare.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.15 247/272] sched/psi: Shut down rtpoll_timer in psi_cgroup_free()
Date: Thu, 20 Aug 2026 16:57:11 +0200
Message-ID: <20260820145239.032589107@linuxfoundation.org>
In-Reply-To: <20260820145231.229664293@linuxfoundation.org>
References: <20260820145231.229664293@linuxfoundation.org>
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5.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 5457025fa8ca3c0d2732109513de839e3e797190 ]

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: stable@vger.kernel.org # v5.10+
Reported-by: Sashiko AI <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260711000434.36C4A1F000E9@smtp.kernel.org/
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Matt Fleming <mfleming@cloudflare.com>
Acked-by: Suren Baghdasaryan <surenb@google.com>
[ adapted `cgroup->psi->` pointer dereference to embedded `cgroup->psi.` and renamed `rtpoll_*` members back to `poll_*` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/sched/psi.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -972,6 +972,12 @@ void psi_cgroup_free(struct cgroup *cgro
 		return;
 
 	cancel_delayed_work_sync(&cgroup->psi.avgs_work);
+	/*
+	 * A psi_schedule_poll_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.poll_timer);
 	free_percpu(cgroup->psi.pcpu);
 	/* All triggers must be removed by now */
 	WARN_ONCE(cgroup->psi.poll_states, "psi: trigger leak\n");