Re: [RFC] IB/srp: Remove create_workqueue

[Bart Van Assche]

On 06/20/2016 08:59 PM, Tejun Heo wrote:
> On Tue, Jun 07, 2016 at 01:00:13PM -0700, Bart Van Assche wrote:
> > > > > srp_remove_wq is used for SRP target port removal work only. This work is
> > > > > neither queued from inside a shrinker nor by the page writeback code so I
> > > > > think it is safe to drop WQ_MEM_RECLAIM.
> > > >
> > > > It should be able to use system_wq then.
> > >
> > > No. I have tried that but that resulted in a deadlock.
> >
> > See also commit bcc059103591 for the details.
>
> So, create_workqueue() limits concurrency to 1 per cpu and if you have
> a dependency between two work items and they get scheduled on the same
> cpu they can deadlock.  system_wq doesn't have that restriction and
> should be fine, AFAICS.

Agreed, as long as WQ_DFL_ACTIVE is not reduced from its current value 
(256) to a very low value (e.g. 1 or 2). This assumption should be 
documented but I'm not sure what the best way is to document this...

Bart.