Re: [PATCH] tick/sched: Do not mess with an enqueued hrtimer

[Frederic Weisbecker]

On Tue, Apr 24, 2018 at 09:22:18PM +0200, Thomas Gleixner wrote:
> Kaike reported that in tests rdma hrtimers occasionaly stopped working. He
> did great debugging, which provided enough context to decode the problem.
> 
> CPU 3			     	      	     CPU 2
> 
> idle
> start sched_timer expires = 712171000000
>  queue->next = sched_timer
> 					    start rdmavt timer. expires = 712172915662
> 					    lock(baseof(CPU3))
> tick_nohz_stop_tick()
> tick = 716767000000			    timerqueue_add(tmr)
> 
> hrtimer_set_expires(sched_timer, tick);
>   sched_timer->expires = 716767000000  <---- FAIL
> 					     if (tmr->expires < queue->next->expires)
> hrtimer_start(sched_timer)		          queue->next = tmr;
> lock(baseof(CPU3))
> 					     unlock(baseof(CPU3))
> timerqueue_remove()
> timerqueue_add()
> 
> ts->sched_timer is queued and queue->next is pointing to it, but then
> ts->sched_timer.expires is modified.
> 
> This not only corrupts the ordering of the timerqueue RB tree, it also
> makes CPU2 see the new expiry time of timerqueue->next->expires when
> checking whether timerqueue->next needs to be updated. So CPU2 sees that
> the rdma timer is earlier than timerqueue->next and sets the rdma timer as
> new next.
> 
> Depending on whether it had also seen the new time at RB tree enqueue, it
> might have queued the rdma timer at the wrong place and then after removing
> the sched_timer the RB tree is completely hosed.
> 
> The problem was introduced with a commit which tried to solve inconsistency
> between the hrtimer in the tick_sched data and the underlying hardware
> clockevent. It split out hrtimer_set_expires() to store the new tick time
> in both the NOHZ and the NOHZ + HIGHRES case, but missed the fact that in
> the NOHZ + HIGHRES case the hrtimer might still be queued.
> 
> Use hrtimer_start(timer, tick...) for the NOHZ + HIGHRES case which sets
> timer->expires after canceling the timer and move the hrtimer_set_expires()
> invocation into the NOHZ only code path which is not affected as it merily
> uses the hrtimer as next event storage so code pathes can be shared with
> the NOHZ + HIGHRES case.
> 
> Fixes: d4af6d933ccf ("nohz: Fix spurious warning when hrtimer and clockevent get out of sync")
> Reported-by: "Wan Kaike" <kaike.wan@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Acked-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Thanks!