From f73f64d5687192bc8eb7f3d9521ca6256b79f224 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Tue, 17 Nov 2020 14:19:43 +0100 Subject: [PATCH] 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: Thomas Gleixner Link: https://lore.kernel.org/r/20201117132006.061341507@linutronix.de --- kernel/time/tick-broadcast.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c index 36d7464..2a47c8f 100644 --- a/kernel/time/tick-broadcast.c +++ b/kernel/time/tick-broadcast.c @@ -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 { -- 2.7.4