[patch 01/20] posix-timers: Prevent RT livelock in itimer_delete()

Thomas Gleixner
2023-04-25T18:49:13Z

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. On RT kernels this is a potential livelock when the exiting task
preempted the hrtimer soft interrupt.

This only affects hrtimer based timers as Posix CPU timers cannot be
concurrently expired. For CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y this is
obviously impossible as the task cannot run task work and exit at the same
time. The CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (only non-RT) is prevented
because interrupts are disabled.

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>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
---
kernel/time/posix-timers.c | 50 +++++++++++++++++++++++++++++++++++++--------
1 file changed, 42 insertions(+), 8 deletions(-)

--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -1037,27 +1037,59 @@ SYSCALL_DEFINE1(timer_delete, timer_t, t
}

/*
- * 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;

+retry_delete:
+ /*
+ * irqsave is required to make timer_wait_running() work.
+ */
+ spin_lock_irqsave(&timer->it_lock, flags);
+
+ /*
+ * 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.
+ *
+ * The CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y case is
+ * irrelevant here because obviously the exiting task
+ * cannot be expiring timer in task work concurrently.
+ * Ditto for CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n as the
+ * tick interrupt cannot run on this CPU because the above
+ * spin_lock disabled interrupts.
+ *
+ * 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 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 +1099,12 @@ void exit_itimers(struct task_struct *ts
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);