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

[Lukas Wunner]

On Mon, Feb 12, 2018 at 05:50:12PM +, Chris Wilson wrote:
> 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 
> > > Cc: Ben Skeggs 
> > > Cc: Alex Deucher 
> > > Signed-off-by: Lukas Wunner 
> > > ---
> > >  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.
> > > + */
> > > +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?

The work_struct exists as long this kthread is working on it.
Since this is called by the kthread itself, it is guaranteed to exist.
So there's no protection needed.

Thanks,

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

[Chris Wilson]

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 
> > Cc: Ben Skeggs 
> > Cc: Alex Deucher 
> > Signed-off-by: Lukas Wunner 
> > ---
> >  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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