Re: softlockups in multi_cpu_stop

[Ming Lei]

On Sat, Mar 7, 2015 at 10:56 AM, Jason Low <jason.low2@xxxxxx> wrote:
> On Sat, 2015-03-07 at 10:10 +0800, Ming Lei wrote:
>> On Sat, Mar 7, 2015 at 10:07 AM, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
>> > On Sat, 2015-03-07 at 09:55 +0800, Ming Lei wrote:
>> >> On Fri, 06 Mar 2015 14:15:37 -0800
>> >> Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
>> >>
>> >> > On Fri, 2015-03-06 at 13:12 -0800, Jason Low wrote:
>> >> > > In owner_running() there are 2 conditions that would make it return
>> >> > > false: if the owner changed or if the owner is not running. However,
>> >> > > that patch continues spinning if there is a "new owner" but it does not
>> >> > > take into account that we may want to stop spinning if the owner is not
>> >> > > running (due to getting rescheduled).
>> >> >
>> >> > So you're rationale is that we're missing this need_resched:
>> >> >
>> >> >     while (owner_running(sem, owner)) {
>> >> >             /* abort spinning when need_resched */
>> >> >             if (need_resched()) {
>> >> >                     rcu_read_unlock();
>> >> >                     return false;
>> >> >             }
>> >> >     }
>> >> >
>> >> > Because the owner_running() would return false, right? Yeah that makes
>> >> > sense, as missing a resched is a bug, as opposed to our heuristics being
>> >> > so painfully off.
>> >> >
>> >> > Sasha, Ming (Cc'ed), does this address the issues you guys are seeing?
>> >>
>> >> For the xfstest lockup, what matters is that the owner isn't running, since
>> >> the following simple change does fix the issue:
>> >
>> > I much prefer Jason's approach, which should also take care of the
>> > issue, as it includes the !owner->on_cpu stop condition to stop
>> > spinning.
>>
>> But the check on owner->on_cpu should be moved outside the loop
>> because new owner can be scheduled out too, right?
>
> We should keep the owner->on_cpu check inside the loop, otherwise we
> could continue spinning if the owner is not running.

So how about checking in this way outside the loop for avoiding the spin?

      if (owner)
           return owner->on_cpu;

Thanks,
Ming Lei
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