From 894a9300d7fb2e2951da92e565ae6de7ddfb0a69 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 18 Aug 2026 15:30:56 -0400 Subject: [PATCH] sched/psi: Shut down rtpoll_timer in psi_cgroup_free() [ 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 Closes: https://lore.kernel.org/all/20260711000434.36C4A1F000E9@smtp.kernel.org/ Signed-off-by: Tejun Heo Acked-by: Johannes Weiner Tested-by: Matt Fleming Acked-by: Suren Baghdasaryan [ adapted `cgroup->psi->` pointer dereference to embedded `cgroup->psi.` and renamed `rtpoll_*` members back to `poll_*` ] Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- kernel/sched/psi.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index fa88bf6ccce0..a7bac5fe2994 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -972,6 +972,12 @@ void psi_cgroup_free(struct cgroup *cgroup) 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");