Re: [PATCH drm-misc-next v5 4/6] drm/gpuvm: track/lock/validate external/evicted objects

[Danilo Krummrich <dakr@redhat.com>]

On 10/4/23 17:29, Thomas Hellström wrote:
> 
> On Wed, 2023-10-04 at 14:57 +0200, Danilo Krummrich wrote:
>> On 10/3/23 11:11, Thomas Hellström wrote:
>>
>> <snip>
>>
>>>>> +
>>>>> +/**
>>>>> + * drm_gpuvm_bo_evict() - add / remove a &drm_gpuvm_bo to /
>>>>> from the &drm_gpuvms
>>>>> + * evicted list
>>>>> + * @vm_bo: the &drm_gpuvm_bo to add or remove
>>>>> + * @evict: indicates whether the object is evicted
>>>>> + *
>>>>> + * Adds a &drm_gpuvm_bo to or removes it from the &drm_gpuvms
>>>>> evicted list.
>>>>> + */
>>>>> +void
>>>>> +drm_gpuvm_bo_evict(struct drm_gpuvm_bo *vm_bo, bool evict)
>>>>> +{
>>>>> +    struct drm_gem_object *obj = vm_bo->obj;
>>>>> +
>>>>> +    dma_resv_assert_held(obj->resv);
>>>>> +
>>>>> +    /* Always lock list transactions, even if
>>>>> DRM_GPUVM_RESV_PROTECTED is
>>>>> +     * set. This is required to protect multiple concurrent
>>>>> calls to
>>>>> +     * drm_gpuvm_bo_evict() with BOs with different dma_resv.
>>>>> +     */
>>>>
>>>> This doesn't work. The RESV_PROTECTED case requires the evicted
>>>> flag we discussed before. The list is either protected by the
>>>> spinlock or the resv. Otherwise a list add could race with a list
>>>> removal elsewhere.
>>
>> I think it does unless I miss something, but it might be a bit subtle
>> though.
>>
>> Concurrent drm_gpuvm_bo_evict() are protected by the spinlock.
>> Additionally, when
>> drm_gpuvm_bo_evict() is called we hold the dma-resv of the
>> corresponding GEM object.
>>
>> In drm_gpuvm_validate() I assert that we hold *all* dma-resv, which
>> implies that no
>> one can call drm_gpuvm_bo_evict() on any of the VM's objects and no
>> one can add a new
>> one and directly call drm_gpuvm_bo_evict() on it either.
> 
> But translated into how the data (the list in this case) is protected
> it becomes
> 
> "Either the spinlock and the bo resv of a single list item OR the bo
> resvs of all bos that can potentially be on the list",
> 
> while this is certainly possible to assert, any new / future code that
> manipulates the evict list will probably get this wrong and as a result
> the code becomes pretty fragile. I think drm_gpuvm_bo_destroy() already
> gets it wrong in that it, while holding a single resv, doesn't take the
> spinlock.

That's true and I don't like it either. Unfortunately, with the dma-resv
locking scheme we can't really protect the evict list without the
drm_gpuvm_bo::evicted trick properly.

But as pointed out in my other reply, I'm a bit worried about the
drm_gpuvm_bo::evicted trick being too restrictive, but maybe it's fine
doing it in the RESV_PROTECTED case.

> 
> So I think that needs fixing, and if keeping that protection I think it
> needs to be documented with the list member and ideally an assert. But
> also note that lockdep_assert_held will typically give false true for
> dma_resv locks; as long as the first dma_resv lock locked in a drm_exec
> sequence  remains locked, lockdep thinks *all* dma_resv locks are held.
> (or something along those lines), so the resv lockdep asserts are
> currently pretty useless.
> 
> /Thomas
> 
> 
> 
>>
>>>>
>>>> Thanks,
>>>>
>>>> Thomas
>>>>
>>>>
>>>
>>
>