Commit f73f64d5 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

tick/broadcast: Serialize access to tick_next_period 



tick_broadcast_setup_oneshot() accesses tick_next_period twice without any
serialization. This is wrong in two aspects:

  - Reading it twice might make the broadcast data inconsistent if the
    variable is updated concurrently.

  - On 32bit systems the access might see an partial update

Protect it with jiffies_lock. That's safe as none of the callchains leading
up to this function can create a lock ordering violation:

timer interrupt
  run_local_timers()
    hrtimer_run_queues()
      hrtimer_switch_to_hres()
        tick_init_highres()
	  tick_switch_to_oneshot()
	    tick_broadcast_switch_to_oneshot()
or
     tick_check_oneshot_change()
       tick_nohz_switch_to_nohz()
         tick_switch_to_oneshot()
           tick_broadcast_switch_to_oneshot()

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201117132006.061341507@linutronix.de
parent 66981c37

kernel/time/tick-broadcast.c

+20 −3
Original line number Diff line number Diff line
@@ -877,6 +877,22 @@ static void tick_broadcast_init_next_event(struct cpumask *mask, 
	}

}



static inline ktime_t tick_get_next_period(void)

{

	ktime_t next;



	/*

	 * Protect against concurrent updates (store /load tearing on

	 * 32bit). It does not matter if the time is already in the

	 * past. The broadcast device which is about to be programmed will

	 * fire in any case.

	 */

	raw_spin_lock(&jiffies_lock);

	next = tick_next_period;

	raw_spin_unlock(&jiffies_lock);

	return next;

}



/**

 * tick_broadcast_setup_oneshot - setup the broadcast device

 */
@@ -905,10 +921,11 @@ static void tick_broadcast_setup_oneshot(struct clock_event_device *bc) 
			   tick_broadcast_oneshot_mask, tmpmask);



		if (was_periodic && !cpumask_empty(tmpmask)) {

			ktime_t nextevt = tick_get_next_period();



			clockevents_switch_state(bc, CLOCK_EVT_STATE_ONESHOT);

			tick_broadcast_init_next_event(tmpmask,

						       tick_next_period);

			tick_broadcast_set_event(bc, cpu, tick_next_period);

			tick_broadcast_init_next_event(tmpmask, nextevt);

			tick_broadcast_set_event(bc, cpu, nextevt);

		} else

			bc->next_event = KTIME_MAX;

	} else {