[RFC PATCH 1/2] dma-fence: Avoid establishing a locking order between fence classes

[Christian König]

Am 30.11.21 um 13:56 schrieb Thomas Hellström:
>
> On 11/30/21 13:42, Christian König wrote:
>> Am 30.11.21 um 13:31 schrieb Thomas Hellström:
>>> [SNIP]
>>>> Other than that, I didn't investigate the nesting fails enough to 
>>>> say I can accurately review this. :)
>>>
>>> Basically the problem is that within enable_signaling() which is 
>>> called with the dma_fence lock held, we take the dma_fence lock of 
>>> another fence. If that other fence is a dma_fence_array, or a 
>>> dma_fence_chain which in turn tries to lock a dma_fence_array we hit 
>>> a splat.
>>
>> Yeah, I already thought that you constructed something like that.
>>
>> You get the splat because what you do here is illegal, you can't mix 
>> dma_fence_array and dma_fence_chain like this or you can end up in a 
>> stack corruption.
>
> Hmm. Ok, so what is the stack corruption, is it that the 
> enable_signaling() will end up with endless recursion? If so, wouldn't 
> it be more usable we break that recursion chain and allow a more 
> general use?

The problem is that this is not easily possible for dma_fence_array 
containers. Just imagine that you drop the last reference to the 
containing fences during dma_fence_array destruction if any of the 
contained fences is another container you can easily run into recursion 
and with that stack corruption.

That's one of the major reasons I came up with the dma_fence_chain 
container. This one you can chain any number of elements together 
without running into any recursion.

> Also what are the mixing rules between these? Never use a 
> dma-fence-chain as one of the array fences and never use a 
> dma-fence-array as a dma-fence-chain fence?

You can't add any other container to a dma_fence_array, neither other 
dma_fence_array instances nor dma_fence_chain instances.

IIRC at least technically a dma_fence_chain can contain a 
dma_fence_array if you absolutely need that, but Daniel, Jason and I 
already had the same discussion a while back and came to the conclusion 
to avoid that as well if possible.

Regards,
Christian.

>
> /Thomas
>
>
>
>
>>
>> Regards,
>> Christian.
>>
>>>
>>> But I'll update the commit message with a typical splat.
>>>
>>> /Thomas
>>