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tick/broadcast: Serialize access to tick_next_period

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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: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201117132006.061341507@linutronix.de
This commit is contained in:
Thomas Gleixner 2020-11-17 14:19:43 +01:00
parent 66981c37b3
commit f73f64d568
1 changed files with 20 additions and 3 deletions
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23
kernel/time/tick-broadcast.c
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@ -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 {