Messages in this thread Patch in this message | | | Date | Wed, 4 Nov 2015 20:35:00 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH] timer: add_timer_on() should perform proper migration |
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Tejun,
On Wed, 4 Nov 2015, Tejun Heo wrote:
> Regardless of the previous CPU a timer was on, add_timer_on() > currently simply sets timer->flags to the new CPU. As the caller must > be seeing the timer as idle, this is locally fine, but the timer > leaving the old base while unlocked can lead to race conditions as > follows.
nice detective work. This has been there forever. I really wonder why nobody ever triggered this before.
@stable:
The patch does only apply to kernels >= 4.2. Backport for older kernels is below.
Thanks,
tglx
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--- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -966,13 +966,26 @@ EXPORT_SYMBOL(add_timer); */ void add_timer_on(struct timer_list *timer, int cpu) { - struct tvec_base *base = per_cpu(tvec_bases, cpu); + struct tvec_base *new_base = per_cpu(tvec_bases, cpu); + struct tvec_base *base; unsigned long flags; timer_stats_timer_set_start_info(timer); BUG_ON(timer_pending(timer) || !timer->function); - spin_lock_irqsave(&base->lock, flags); - timer_set_base(timer, base); + + /* + * If @timer was on a different CPU, it should be migrated with the + * old base locked to prevent other operations proceeding with the + * wrong base locked. See lock_timer_base(). + */ + base = lock_timer_base(timer, &flags); + if (base != new_base) { + timer_set_base(timer, NULL); + spin_unlock(&base->lock); + base = new_base; + spin_lock(&base->lock); + timer_set_base(timer, base); + } debug_activate(timer, timer->expires); internal_add_timer(base, timer); spin_unlock_irqrestore(&base->lock, flags);
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