[PATCH 2/5] drm: Allow determining if current task is output poll worker

[Lukas Wunner]

On Mon, Feb 12, 2018 at 12:46:11PM -0500, Lyude Paul wrote:
> On Sun, 2018-02-11 at 10:38 +0100, Lukas Wunner wrote:
> > Introduce a helper to determine if the current task is an output poll
> > worker.
> > 
> > This allows us to fix a long-standing deadlock in several DRM drivers
> > wherein the ->runtime_suspend callback waits for the output poll worker
> > to finish and the worker in turn calls a ->detect callback which waits
> > for runtime suspend to finish.  The ->detect callback is invoked from
> > multiple call sites and waiting for runtime suspend to finish is the
> > correct thing to do except if it's executing in the context of the
> > worker.
[snip]
> > +/**
> > + * drm_kms_helper_is_poll_worker - is %current task an output poll worker?
> > + *
> > + * Determine if %current task is an output poll worker.  This can be used
> > + * to select distinct code paths for output polling versus other contexts.
> > + */
>
> For this, it would be worth explicitly noting in the comments herethat this
> should be called by DRM drivers in order to prevent racing with hotplug
> polling workers, so that new drivers in the future can avoid implementing this
> race condition in their driver.

Good point, I've just sent out a v2 to address your comment.  Let me know
if this isn't what you had in mind.  It may also be worth to expand the
DOC section at the top of drm_probe_helper.c to explain the interaction
between polling and runtime suspend in more detail, but I think this is
better done in a separate patch to keep the present patch small and thus
easily backportable to stable.

Thanks a lot for the review,

Lukas