[Intel-gfx] [PATCH] drm/i915: Ensure associated VMAs are inactive when contexts are destroyed

[Tvrtko Ursulin]

On 26/10/15 11:23, Chris Wilson wrote:
> On Mon, Oct 26, 2015 at 11:05:03AM +0000, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> In the following commit:
>>
>>      commit e9f24d5fb7cf3628b195b18ff3ac4e37937ceeae
>>      Author: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>      Date:   Mon Oct 5 13:26:36 2015 +0100
>>
>>          drm/i915: Clean up associated VMAs on context destruction
>>
>> I added a WARN_ON assertion that VM's active list must be empty
>> at the time of owning context is getting freed, but that turned
>> out to be a wrong assumption.
>>
>> Due ordering of operations in i915_gem_object_retire__read, where
>> contexts are unreferenced before VMAs are moved to the inactive
>> list, the described situation can in fact happen.
>
> The context is being unreferenced indirectly. Adding a direct reference
> here is even more bizarre.

Perhaps is not the prettiest, but it sounds logical to me to ensure that 
order of destruction of involved object hierarchy goes from the 
bottom-up and is not interleaved.

If you consider the active/inactive list position as part of the retire 
process, doing it at the very place in code, and the very object that 
looked to be destroyed out of sequence, to me sounded logical.

How would you do it, can you think of a better way?

Regards,

Tvrtko