[Intel-gfx] [PATCH] drm/i915: Embrace the race in busy-ioctl

[Joonas Lahtinen]

On pe, 2016-08-12 at 18:52 +0100, Chris Wilson wrote:
> Daniel Vetter proposed a new challenge to the serialisation inside the
> busy-ioctl that exposed a flaw that could result in us reporting the
> wrong engine as being busy. If the request is reallocated as we test
> its busyness and then reassigned to this object by another thread, we
> would not notice that the test itself was incorrect.
> 
> We are faced with a choice of using __i915_gem_active_get_request_rcu()
> to first acquire a reference to the request preventing the race, or to
> acknowledge the race and accept the limitations upon the accuracy of the
> busy flags. Note that we guarantee that we never falsely report the
> object as idle (providing userspace itself doesn't race), and so the
> most important use of the busy-ioctl and its guarantees are fulfilled.
> 

If Daniel acks the userspace change,

Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation