[tip: timers/core] posix-timers: Prevent RT livelock in itimer_delete()

tip-bot2 for Thomas Gleixner
2023-06-18T20:51:21Z

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

Commit-ID: 9d9e522010eb5685d8b53e8a24320653d9d4cbbf
Gitweb: https://git.kernel.org/tip/9d9e522010eb5685d8b53e8a24320653d9d4cbbf
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 01 Jun 2023 22:16:34 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sun, 18 Jun 2023 22:40:42 +02:00

posix-timers: Prevent RT livelock in itimer_delete()

itimer_delete() has a retry loop when the timer is concurrently expired. On
non-RT kernels this just spin-waits until the timer callback has completed,
except for posix CPU timers which have HAVE_POSIX_CPU_TIMERS_TASK_WORK
enabled.

In that case and on RT kernels the existing task could live lock when
preempting the task which does the timer delivery.

Replace spin_unlock() with an invocation of timer_wait_running() to handle
it the same way as the other retry loops in the posix timer code.

Fixes: ec8f954a40da ("posix-timers: Use a callback for cancel synchronization on PREEMPT_RT")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/87v8g7c50d.ffs@tglx
---
kernel/time/posix-timers.c | 43 ++++++++++++++++++++++++++++++-------
1 file changed, 35 insertions(+), 8 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 808a247..ed3c4a9 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -1037,27 +1037,52 @@ retry_delete:
}

/*
- * return timer owned by the process, used by exit_itimers
+ * Delete a timer if it is armed, remove it from the hash and schedule it
+ * for RCU freeing.
*/
static void itimer_delete(struct k_itimer *timer)
{
-retry_delete:
- spin_lock_irq(&timer->it_lock);
+ unsigned long flags;
+
+ /*
+ * irqsave is required to make timer_wait_running() work.
+ */
+ spin_lock_irqsave(&timer->it_lock, flags);

+retry_delete:
+ /*
+ * Even if the timer is not longer accessible from other tasks
+ * it still might be armed and queued in the underlying timer
+ * mechanism. Worse, that timer mechanism might run the expiry
+ * function concurrently.
+ */
if (timer_delete_hook(timer) == TIMER_RETRY) {
- spin_unlock_irq(&timer->it_lock);
+ /*
+ * Timer is expired concurrently, prevent livelocks
+ * and pointless spinning on RT.
+ *
+ * timer_wait_running() drops timer::it_lock, which opens
+ * the possibility for another task to delete the timer.
+ *
+ * That's not possible here because this is invoked from
+ * do_exit() only for the last thread of the thread group.
+ * So no other task can access and delete that timer.
+ */
+ if (WARN_ON_ONCE(timer_wait_running(timer, &flags) != timer))
+ return;
+
goto retry_delete;
}
list_del(&timer->list);

- spin_unlock_irq(&timer->it_lock);
+ spin_unlock_irqrestore(&timer->it_lock, flags);
release_posix_timer(timer, IT_ID_SET);
}

/*
- * This is called by do_exit or de_thread, only when nobody else can
- * modify the signal->posix_timers list. Yet we need sighand->siglock
- * to prevent the race with /proc/pid/timers.
+ * Invoked from do_exit() when the last thread of a thread group exits.
+ * At that point no other task can access the timers of the dying
+ * task anymore.
*/
void exit_itimers(struct task_struct *tsk)
{
@@ -1067,10 +1092,12 @@ void exit_itimers(struct task_struct *tsk)
if (list_empty(&tsk->signal->posix_timers))
return;

+ /* Protect against concurrent read via /proc/$PID/timers */
spin_lock_irq(&tsk->sighand->siglock);
list_replace_init(&tsk->signal->posix_timers, &timers);
spin_unlock_irq(&tsk->sighand->siglock);

+ /* The timers are not longer accessible via tsk::signal */
while (!list_empty(&timers)) {
tmr = list_first_entry(&timers, struct k_itimer, list);
itimer_delete(tmr);