On Tuesday, December 08, 2015 12:26:22 PM Viresh Kumar wrote: > On 07-12-15, 23:43, Rafael J. Wysocki wrote: > > On Monday, December 07, 2015 01:20:27 PM Viresh Kumar wrote: > > > > At this point we might end up decrementing skip_work from > > > gov_cancel_work() and then cancel the work which we haven't queued > > > yet. And the end result will be that the work is still queued while > > > gov_cancel_work() has finished. > > > > I'm not quite sure how that can happen. > > I will describe that towards the end of this email. > > > There is a bug in this code snippet, but it may cause us to fail to queue > > the work at all, so the incrementation and the check need to be done > > under the spinlock. > > What bug ?
Well, if the timer function runs on all CPUs at the same time, they all can see skip_work > 1 and none of them will queue the work. > > > And we have to keep the atomic operation, as well as queue_work() > > > within the lock. > > > > Putting queue_work() under the lock doesn't prevent any races from > > happening, > > Then I am not able to think about it properly, but I will at least > present my case here :) > > > because only one of the CPUs can execute that part of the function anyway. > > > > > > queue_work(system_wq, &shared->work); > > > > > > > > and the remaining incrementation and decrementation of skip_work are > > > > replaced > > > > with the corresponding atomic operations, it still should work, no? > > > > Well, no, the above wouldn't work. > > > > But what about something like this instead: > > > > if (atomic_inc_return(&shared->skip_work) > 1) > > atomic_dec(&shared->skip_work); > > else > > queue_work(system_wq, &shared->work); > > > > (plus the changes requisite replacements in the other places)? > > > > Only one CPU can see the result of the atomic_inc_return() as 1 and this is > > the > > only one that will queue up the work item, unless I'm missing anything super > > subtle. > > Looks like you are talking about the race between different timer > handlers, which race against queuing the work. Sorry if you are not. > But I am not talking about that thing.. > > Suppose queue_work() isn't done within the spin lock. > > CPU0 CPU1 > > cpufreq_governor_stop() dbs_timer_handler() > -> gov_cancel_work() -> lock > -> shared->skip_work++, as > skip_work was 0. //skip_work=1 > -> unlock > -> lock > -> shared->skip_work++; //skip_work=2 > -> unlock > -> cancel_work_sync(&shared->work); > -> queue_work(); > -> gov_cancel_timers(shared->policy); > -> shared->skip_work = 0; > dbs_work_handler(); > > > > And according to how I understand it, we are screwed up at this point. > And its the same old bug which I fixed recently (which we hacked up by > using gov-lock earlier). You are right, I've overlooked that race (but then it is rather easy to overlook). Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
