[Intel-gfx] [PATCH 1/3] drm/i915/gt: Do not allow setting ring size for legacy ring submission

[Maarten Lankhorst]

Op 21-06-2021 om 14:52 schreef Tvrtko Ursulin:
>
> On 21/06/2021 13:08, Tvrtko Ursulin wrote:
>>
>> I had some questions on the trybot mailing list, let me copy&paste..
>>
>> On 21/06/2021 12:41, Maarten Lankhorst wrote:
>>> It doesn't work for legacy ring submission, and is in the best case
>>> ignored.
>>
>> Looks rejected instead of ignored:
>>
>> static int set_ringsize(struct i915_gem_context *ctx,
>>              struct drm_i915_gem_context_param *args)
>> {
>>      if (!HAS_LOGICAL_RING_CONTEXTS(ctx->i915))
>>          return -ENODEV;
>>>
>>> In the worst case we end up freeing engine->legacy.ring for all other
>>> active engines, resulting in a use-after-free.
>>
>> Worst case is cloning because ring_context_alloc is not taking a reference to engine->legacy.ring, or something else?
>
> No can't be that, it was my incomplete analysis last week. Since ring_context_destroy does not actually free the legacy ring I don't see any use after free paths.
>
> Regards,

Hmm, it gets stuck inside intel_context_set_ring_size when cloning engines..

I guess it can't happen in practice, just the code introduces the race by preallocating
inside intel_context_lock_pinned()..

copy_ring_size() should only be called for HAS_LOGICAL_RING_CONTEXTS().
I guess that makes this patch obsolete. It can safely be dropped from the series,
I think I should probably introduce a check to only set the size when HAS_LOGICAL_RING_CONTEXTS
evaluates to true, but that wouldn't block the rest of this series.

~Maarten