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: ec8f954a ("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

@@ -1037,27 +1037,52 @@ SYSCALL_DEFINE1(timer_delete, timer_t, timer_id)
 }
 
 /*
- * 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);