Re: [PATCH v2] RFC drm/i915: Mark runtime_pm as a special class of lock

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jul 12, 2018 at 09:41:07AM +0100, Chris Wilson wrote:
> Quoting Chris Wilson (2018-07-12 09:36:33)
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/i915_drv.c         |  5 +++++
> >  drivers/gpu/drm/i915/i915_drv.h         |  1 +
> >  drivers/gpu/drm/i915/intel_runtime_pm.c | 11 +++++++++++
> >  3 files changed, 17 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> > index 3eba3d1ab5b8..2e6d3259f6d0 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.c
> > +++ b/drivers/gpu/drm/i915/i915_drv.c
> > @@ -2603,6 +2603,7 @@ static int intel_runtime_suspend(struct device *kdev)
> >         DRM_DEBUG_KMS("Suspending device\n");
> >  
> >         disable_rpm_wakeref_asserts(dev_priv);
> > +       lock_map_acquire(&dev_priv->runtime_pm.lock);
> >  
> >         /*
> >          * We are safe here against re-faults, since the fault handler takes
> > @@ -2637,11 +2638,13 @@ static int intel_runtime_suspend(struct device *kdev)
> >                 i915_gem_init_swizzling(dev_priv);
> >                 i915_gem_restore_fences(dev_priv);
> >  
> > +               lock_map_release(&dev_priv->runtime_pm.lock);
> >                 enable_rpm_wakeref_asserts(dev_priv);
> >  
> >                 return ret;
> >         }
> >  
> > +       lock_map_release(&dev_priv->runtime_pm.lock);
> 
> What happens if we don't release the lock here? I think that's what we
> want... While suspended we are not allowed to do any action that would
> ordinarily require a wakeref. However that scares me for being both
> incredibly broad, and that I think lockdep is process centric so doesn't
> track locks in this manner?

Lockdep requires that acquire&release are in the same process context. For
dependencies crossing boundaries we want a cross-release. And yes I think
a cross-release dependency between our rpm_suspend and rpm_get is required
for full anotation. But since cross-release is suffering in limbo due to
meltdown/spectre that's a way off still :-/

Also I think if this all works out we should propose it as a patch to core
rpm code (maybe once the cross-release stuff has landed too).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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