Re: Should suspend plug low-level devices?

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Mar 17, 2016 at 02:51:36PM -0400, Alan Stern wrote:
> > Stopping kthreads in itself is fine but the problem is that we
> > currently don't know who's stopping what.  Just making each freezing
> > explicit, say, an explicit invocation of kthread_freeze_kthread(),
> > wouldn't be that difficult and would go a long way.  e.g. It would
> > immediately solve the problem arising from kthread freezing order not
> > matching device dependency hierarchy.
> 
> We're talking about workqueues in particular.  Freezing order is not 
> the issue.

Whether something is running on kthread or workqueue doesn't make any
difference.  Why would it?  It's about dependency chain, not what
something is running on.

> > * Make block layer plug bio's from upper layers.
> > 
> > * Visit each freezable workqueue or kthread users and convert them to
> >   plug explicitly.
> 
> How would this help the case in question?  If a workqueue is plugged 
> instead of frozen, won't flush_delayed_work still deadlock?

By plugging and unplugging in a well defined order rather than
"whatever comes first in workqueue or task lists".

> kthreads (including workqueues) get frozen at controlled places.  
> Presumably they are smart enough not to allow themselves to be frozen
> while any I/Os are in flight.

I don't think that's the main problem here.

Thanks.

-- 
tejun