Re: softlockups in multi_cpu_stop

Davidlohr Bueso
2015-03-07T02:07:46Z

On Sat, 2015-03-07 at 09:55 +0800, Ming Lei wrote:
> On Fri, 06 Mar 2015 14:15:37 -0800
> Davidlohr Bueso <dave@stgolabs.net> 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.

>
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 06e2214..5e08705 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -358,8 +358,9 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
> }
> rcu_read_unlock();
>
> - if (READ_ONCE(sem->owner))
> - return true; /* new owner, continue spinning */
> + owner = READ_ONCE(sem->owner);
> + if (owner && owner->on_cpu)
> + return true;
>
> /*
> * When the owner is not set, the lock could be free or
>
>
> Thanks,
> Ming Lei