Am 30.11.21 um 15:35 schrieb Thomas Hellström:

On Tue, 2021-11-30 at 14:26 +0100, Christian König wrote:

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Am 30.11.21 um 13:56 schrieb Thomas Hellström:

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On 11/30/21 13:42, Christian König wrote:

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Am 30.11.21 um 13:31 schrieb Thomas Hellström:

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[SNIP]

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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.

Indeed, that would require some deeper surgery.

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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.

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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.

Yes, this is actually the use-case. But what I can't easily guarantee is that that dma_fence_chain isn't fed into a dma_fence_array somewhere else. How do you typically avoid that?

Meanwhile I guess I need to take a different approach in the driver to avoid this altogether.

Jason and I came up with a deep dive iterator for his use case, but I think we don't want to use that any more after my dma_resv rework.

In other words when you need to create a new dma_fence_array you flatten out the existing construct which is at worst case dma_fence_chain->dma_fence_array->dma_fence.

Regards, Christian.

/Thomas

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Regards, Christian.

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/Thomas

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Regards, Christian.

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But I'll update the commit message with a typical splat.

/Thomas