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

[Chris Wilson]

On Wed, Dec 23, 2015 at 11:32:23AM +0000, Chris Wilson wrote:
> On Wed, Dec 23, 2015 at 11:19:23AM +0000, 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.
> > 
> > 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.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> 
> Some food for thought. I would especially like someone to poke holes in
> the racy pointer lookup and check the store mb() versus the
> rcu-reference.

So what I think is the missing element here is then

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 871aaae1a9d5..4d4ab8e6423f 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4193,7 +4193,8 @@ i915_gem_load(struct drm_device *dev)
        dev_priv->requests =
                kmem_cache_create("i915_gem_request",
                                  sizeof(struct drm_i915_gem_request), 0,
-                                 SLAB_HWCACHE_ALIGN,
+                                 SLAB_HWCACHE_ALIGN |
+                                 SLAB_DESTROY_BY_RCU,
                                  NULL);
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre