linux-kernel - Re: [PATCH v2 4/4] dma-buf: Check status of enable-signaling bit on debug

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Date:   Tue, 6 Sep 2022 12:43:39 +0200
From:   Christian König <christian.koenig@....com>
To:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Arvind Yadav <Arvind.Yadav@....com>, andrey.grodzovsky@....com,
        shashank.sharma@....com, amaranath.somalapuram@....com,
        Arunpravin.PaneerSelvam@....com, sumit.semwal@...aro.org,
        gustavo@...ovan.org, airlied@...ux.ie, daniel@...ll.ch,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] dma-buf: Check status of enable-signaling bit on
 debug

Am 06.09.22 um 12:20 schrieb Tvrtko Ursulin:
>
> On 06/09/2022 09:39, Christian König wrote:
>> Am 05.09.22 um 18:35 schrieb Arvind Yadav:
>>> The core DMA-buf framework needs to enable signaling
>>> before the fence is signaled. The core DMA-buf framework
>>> can forget to enable signaling before the fence is signaled.
>>
>> This sentence is a bit confusing. I'm not a native speaker of English 
>> either, but I suggest something like:
>>
>> "Fence signaling must be enable to make sure that the 
>> dma_fence_is_signaled() function ever returns true."
>>
>>> To avoid this scenario on the debug kernel, check the
>>> DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT status bit before checking
>>> the signaling bit status to confirm that enable_signaling
>>> is enabled.
>>
>> This describes the implementation, but we should rather describe the 
>> background of the change. The implementation should be obvious. 
>> Something like this maybe:
>>
>> "
>> Since drivers and implementations sometimes mess this up enforce 
>> correct behavior when DEBUG_WW_MUTEX_SLOWPATH is used during debugging.
>>
>> This should make any implementations bugs resulting in not signaled 
>> fences much more obvious.
>> "
>
> I think I follow the idea but am not sure coupling (well "coupling".. 
> not really, but cross-contaminating in a way) dma-fence.c with a 
> foreign and effectively unrelated concept of a ww mutex is the best way.
>
> Instead, how about a dma-buf specific debug kconfig option?

Yeah, I was thinking about that as well.

The slowpath config option was just at hand because when you want to 
test the slowpath you want to test this here as well.

>
> Condition would then be, according to my understanding of the rules 
> and expectations, along the lines of:
>
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 775cdc0b4f24..147a9df2c9d0 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -428,6 +428,17 @@ dma_fence_is_signaled_locked(struct dma_fence 
> *fence)
>  static inline bool
>  dma_fence_is_signaled(struct dma_fence *fence)
>  {
> +#ifdef CONFIG_DEBUG_DMAFENCE
> +       /*
> +        * Implementations not providing the enable_signaling callback 
> are
> +        * required to always have signaling enabled or fences are not
> +        * guaranteed to ever signal.
> +        */

Well that comment is a bit misleading. The intention of the extra check 
is to find bugs in the frontend and not the backend.

In other words somewhere in the drm_syncobj code we have a 
dma_fence_is_signaled() call without matching 
dma_fence_enable_sw_signaling().

Regards,
Christian.

> + if (!fence->ops->enable_signaling &&
> +           !test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags))
> +               return false;
> +#endif
> +
>         if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>                 return true;
>
> Thoughts?
>
> Regards,
>
> Tvrtko
>
>>
>>>
>>> Signed-off-by: Arvind Yadav <Arvind.Yadav@....com>
>>
>> With the improved commit message this patch is Reviewed-by: Christian 
>> König <christian.koenig@....com>
>>
>> Regards,
>> Christian.
>>
>>> ---
>>>
>>> Changes in v1 :
>>> 1- Addressing Christian's comment to replace
>>> CONFIG_DEBUG_WW_MUTEX_SLOWPATH instead of CONFIG_DEBUG_FS.
>>> 2- As per Christian's comment moving this patch at last so
>>> The version of this patch is also changed and previously
>>> it was [PATCH 1/4]
>>>
>>>
>>> ---
>>>   include/linux/dma-fence.h | 5 +++++
>>>   1 file changed, 5 insertions(+)
>>>
>>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>>> index 775cdc0b4f24..ba1ddc14c5d4 100644
>>> --- a/include/linux/dma-fence.h
>>> +++ b/include/linux/dma-fence.h
>>> @@ -428,6 +428,11 @@ dma_fence_is_signaled_locked(struct dma_fence 
>>> *fence)
>>>   static inline bool
>>>   dma_fence_is_signaled(struct dma_fence *fence)
>>>   {
>>> +#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
>>> +    if (!test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags))
>>> +        return false;
>>> +#endif
>>> +
>>>       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>>>           return true;
>>