[Intel-gfx] [PATCH 07/27] drm/i915: Coordinate i915_active with its own mutex

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-09-27 13:25:23)
> 
> On 27/09/2019 13:16, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2019-09-27 13:08:51)
> >>
> >> On 27/09/2019 12:25, Chris Wilson wrote:
> >>> Quoting Tvrtko Ursulin (2019-09-27 12:10:29)
> >>>>
> >>>> On 25/09/2019 11:01, Chris Wilson wrote:
> >>>>> +struct dma_fence *
> >>>>> +__i915_active_fence_set(struct i915_active_fence *active,
> >>>>> +                     struct dma_fence *fence)
> >>>>
> >>>> Can you add a short comment for this function explaining the racyness
> >>>> and so why it returns prev and what should the callers do with it?
> >>>
> >>> Before using this function, we had the callers declare what mutex guards
> >>> this timeline and we check here that is held.
> >>
> >> No, I mean because it has to reload prev and return it to the caller,
> >> which implies that is to handle some designed-in racy-ness which I think
> >> should be mentioned.
> > 
> > But that's not racing with the caller, that just racing with the
> > callback from the interrupt handler and the reason why we have to check
> > after serialising is called out. /* serialise with prev->cb_list */ ?
> > 
> > The caller is responsible for ensuring that prev is executed before
> > fence to keep the timeline in the same order as recorded.
> 
> I did not say it is racing with the caller, but that the caller has to 
> handle a race.

But the caller only has to care about "was there already an active fence
on this timeline? If so, I must make sure it executes before me and take
that into consideration for the flow along the timeline"

I'm not seeing what race the caller has to be concerned with here. If
they replace the last request in the timeline, they have it returned.
If there was no request previously in the timeline, they have NULL.
(That just seems straightforward.)
 
> "Serialise with prev->cb_list" is too low level for me. Trust me, I 
> always struggle with the active tracking code since there is so many 
> indirections and relationships. So in the absence of a visual diagram a 
> higher level comment would be beneficial for the future self. Just 
> expanding on what you replied here talking about how interrupts 
> interacts with new stuff entering tracking would do it.

It's just about that the callback may be executing and so we need to
serialise the list manipulation under the lock; having performed that
list manipulation, we then know whether or not we were successful in
capturing the previous fence.
-Chris