Re: [PATCH v3] drm/i915: Add to timeline requires the timeline mutex

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Chris Wilson (2019-07-08 13:22:01)
> Quoting Tvrtko Ursulin (2019-07-08 13:18:02)
> > 
> > On 08/07/2019 12:39, Chris Wilson wrote:
> > > v2: Though it doesn't affect the current users, contexts may share
> > > timelines so check if we already hold the right mutex.
> 
> > > +int intel_context_prepare_remote_request(struct intel_context *ce,
> > > +                                      struct i915_request *rq)
> > > +{
> > > +     struct intel_timeline *tl = ce->ring->timeline;
> > > +     int err;
> > > +
> > > +     /* Only suitable for use in remotely modifying this context */
> > > +     GEM_BUG_ON(rq->hw_context == ce);
> > > +
> > > +     if (rq->timeline != tl) { /* beware timeline sharing */
> > > +             err = mutex_lock_interruptible_nested(&tl->mutex,
> > > +                                                   SINGLE_DEPTH_NESTING);
> > 
> > Which caller is holding tl->mutex?
> 
> None today, but it is conceivable. So rather than have a mysterious
> deadlock not reported by lockdep in the future, nip the problem in the
> bud.

Just in case, creating a request takes the timeline->mutex and holds it
until submitted. So the scenario I considered was if one user context
wanted to modify another one (which could work), but they happen to be
on a common timeline.
-Chris
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