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

Chris Wilson
2018-02-12T17:50:12Z

Quoting Lyude Paul (2018-02-12 17:46:11)
> 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.
> >
> > Cc: Dave Airlie <airlied@redhat.com>
> > Cc: Ben Skeggs <bskeggs@redhat.com>
> > Cc: Alex Deucher <alexander.deucher@amd.com>
> > Signed-off-by: Lukas Wunner <lukas@wunner.de>
> > ---
> > drivers/gpu/drm/drm_probe_helper.c | 14 ++++++++++++++
> > include/drm/drm_crtc_helper.h | 1 +
> > 2 files changed, 15 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_probe_helper.c
> > b/drivers/gpu/drm/drm_probe_helper.c
> > index 555fbe54d6e2..019881d15ce1 100644
> > --- a/drivers/gpu/drm/drm_probe_helper.c
> > +++ b/drivers/gpu/drm/drm_probe_helper.c
> > @@ -653,6 +653,20 @@ static void output_poll_execute(struct work_struct
> > *work)
> > schedule_delayed_work(delayed_work,
> > DRM_OUTPUT_POLL_PERIOD);
> > }
> >
> > +/**
> > + * 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.
>
> > +bool drm_kms_helper_is_poll_worker(void)
> > +{
> > + struct work_struct *work = current_work();
> > +
> > + return work && work->func == output_poll_execute;

What ensures that work is accessible? Does this need rcu_read_lock
protection or more?
-Chris
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