Subject: Re: deadlocks if use htb

From: Jarek Poplawski <[email-masked]>
Date: Thu, 15 Jan 2009 10:54:17 +0000

On Thu, Jan 15, 2009 at 11:46:48AM +0100, Peter Zijlstra wrote:
> On Thu, 2009-01-15 at 09:01 +0000, Jarek Poplawski wrote:
...
> > spin_lock
> > (not this hrtimer's anymore)
> > __remove_hrtimer
> > list_add_tail			enqueue_hrtimer
> > 
> 
> (looking at .28 code)
> 
> run_hrtimer_pending() reads like:
> 
> while (pending timers) {
>   __remove_hrtimer(timer, HRTIMER_STATE_CALLBACK);
>   spin_unlock(&cpu_base->lock);
> 
>   fn(timer);
> 
>   spin_lock(&cpu_base->lock);
>   timer->state &= ~HRTIMER_STATE_CALLBACK; // _should_ result in HRTIMER_STATE_INACTIVE
>   if (HRTIMER_RESTART)
>     re-queue
>   else if (timer->state != INACTIVE) {
>     // so another cpu re-queued this timer _while_ we were executing it.
>     if (timer is first && !reprogramm) {
>       __remove_hrtimer(timer, HRTIMER_STATE_PENDING);
>       list_add_tail(timer, &cb_pending);
>     }
>   } 
> }
> 
> So in the window where we drop the lock, one can, as you said, have
> another cpu requeue the timer, but the rb_entry and list_entry are free,
> so it should not cause the data corruption we're seeing.
> 

Can't they be enqueued to the list (without a lock) and rbtree at the
same time? Then removing is done for the list only?

Jarek P.