On Tue, 5 Apr 2016 21:24:24 +0200 luca abeni <[email protected]> wrote:
> On Tue, 5 Apr 2016 20:02:52 +0200 > Peter Zijlstra <[email protected]> wrote: > > > On Tue, Apr 05, 2016 at 07:56:57PM +0200, luca abeni wrote: > > > > > > > + migrate_active = hrtimer_active(&p->dl.inactive_timer); > > > > > + if (migrate_active) > > > > > + sub_running_bw(&p->dl, &rq->dl); > > > > > + raw_spin_unlock(&rq->lock); > > > > > > > > At this point task_rq() is still the above rq, so if the inactive timer > > > > hits here it will lock this rq and subtract the running bw here _again_, > > > > right? > > > I think it will see the task state as TASK_RUNNING, so it will do nothing. > > > Or it will cancelled later when the task is enqueued... I'll double check > > > this. > > > > Right, so this is select_task_rq_dl(), we run this in wakeups, before > > TASK_RUNNING. > > Sigh... I knew I was missing something here... :( > So, I think the solution here is to use double_lock_balance() (or something > like that) to take both the rq locks so that the inactive timer handler cannot > run between sub_running_bw() and add_running_bw()... I'll try this. Double thinking about this: isn't p->pi_lock saving us here? I mean: - try_to_wake_up() takes p->pi_lock before doing anything else - so, select_task_rq() is invoked with p->pi_lock locked - but inactive_task_timer() does "rq = task_rq_lock(p, &flags)", and task_rq_lock() tries to take p->pi_lock - so, we should be safe, no? Maybe this is why I never managed to trigger this race... :) Thanks, Luca
