[Intel-gfx] [PATCH 10/16] drm/i915: Remove (struct_mutex) locking for wait-ioctl

[Chris Wilson]

On Thu, Aug 04, 2016 at 11:26:04AM +0300, Joonas Lahtinen wrote:
> On ma, 2016-08-01 at 19:22 +0100, Chris Wilson wrote:
> > With a bit of care (and leniency) we can iterate over the object and
> > wait for previous rendering to complete with judicial use of atomic
> > reference counting. The ABI requires us to ensure that an active object
> > is eventually flushed (like the busy-ioctl) which is guaranteed by our
> > management of requests (i.e. everything that is submitted to hardware is
> > flushed in the same request). All we have to do is ensure that we can
> > detect when the requests are complete for reporting when the object is
> > idle (without triggering ETIME) - this is handled by
> > __i915_wait_request.
> > 
> > The biggest danger in the code is walking the object without holding any
> > locks. We iterate over the set of last requests and carefully grab a
> > reference upon it. (If it is changing beneath us, that is the usual
> > userspace race and even with locking you get the same indeterminate
> > results.) If the request is unreferenced beneath us, it will be disposed
> > of into the request cache - so we have to carefully order the retrieval
> > of the request pointer with its removal, and to do this we employ RCU on
> > the request cache and upon the last_request pointer tracking.
> > 
> > The impact of this is actually quite small - the return to userspace
> > following the wait was already lockless. What we achieve here is
> > completing an already finished wait without hitting the struct_mutex,
> > our hold is quite short and so we are typically just a victim of
> > contention rather than a cause.
> > 
> 
> The commit message seems little bit disconnect with the code, making
> the patch sound much more complex than it is. Is it up to date? Or
> maybe parts of this explanation would belong to an earlier patch?

Well, you caught me. This used to be quite ugly until it was hidden
await behind a new helper, and this code has been through multiple
phases gradually removing the struct_mutex further and further.

With a couple of glaring exceptions (the bits not talking about the
innards but about the top level implementation), it is describing what is
going on under the covers.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre