Subject: [PATCH 1/2] posix-timers: Prevent RT livelock in itimer_delete()
From: Thomas Gleixner <[email-masked]>
Date: Tue, 25 Apr 2023 20:48:56 +0200

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 <[email-masked]>
Cc: Sebastian Siewior <[email-masked]>
Link: https://lore.kernel.org/all/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/time/posix-timers.c |   50 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 42 insertions(+), 8 deletions(-)

Index: linux-6.3.0-rt11/kernel/time/posix-timers.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/time/posix-timers.c
+++ linux-6.3.0-rt11/kernel/time/posix-timers.c
@@ -1033,27 +1033,59 @@ 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;
 
+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)
 {
@@ -1063,10 +1095,12 @@ 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);