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Date:   Tue, 5 Dec 2023 16:58:15 +0100
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Subject: Re: [RFC] drm/scheduler: Unwrap job dependencies
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To:     Rob Clark <robdclark@gmail.com>
Cc:     dri-devel@lists.freedesktop.org,
        Rob Clark <robdclark@chromium.org>,
        Luben Tuikov <luben.tuikov@amd.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>
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From:   =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>
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Am 05.12.23 um 16:41 schrieb Rob Clark:
> On Mon, Dec 4, 2023 at 10:46 PM Christian König
> <christian.koenig@amd.com> wrote:
>> Am 04.12.23 um 22:54 schrieb Rob Clark:
>>> On Thu, Mar 23, 2023 at 2:30 PM Rob Clark <robdclark@gmail.com> wrote:
>>>> [SNIP]
>>> So, this patch turns out to blow up spectacularly with dma_fence
>>> refcnt underflows when I enable DRIVER_SYNCOBJ_TIMELINE .. I think,
>>> because it starts unwrapping fence chains, possibly in parallel with
>>> fence signaling on the retire path.  Is it supposed to be permissible
>>> to unwrap a fence chain concurrently?
>> The DMA-fence chain object and helper functions were designed so that
>> concurrent accesses to all elements are always possible.
>>
>> See dma_fence_chain_walk() and dma_fence_chain_get_prev() for example.
>> dma_fence_chain_walk() starts with a reference to the current fence (the
>> anchor of the walk) and tries to grab an up to date reference on the
>> previous fence in the chain. Only after that reference is successfully
>> acquired we drop the reference to the anchor where we started.
>>
>> Same for dma_fence_array_first(), dma_fence_array_next(). Here we hold a
>> reference to the array which in turn holds references to each fence
>> inside the array until it is destroyed itself.
>>
>> When this blows up we have somehow mixed up the references somewhere.
> That's what it looked like to me, but wanted to make sure I wasn't
> overlooking something subtle.  And in this case, the fence actually
> should be the syncobj timeline point fence, not the fence chain.
> Virtgpu has essentially the same logic (there we really do want to
> unwrap fences so we can pass host fences back to host rather than
> waiting in guest), I'm not sure if it would blow up in the same way.

Well do you have a backtrace of what exactly happens?

Maybe we have some _put() before _get() or something like this.

Thanks,
Christian.

>
> BR,
> -R
>
>> Regards,
>> Christian.
>>
>>> BR,
>>> -R