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

[Tvrtko Ursulin]

On 06/09/2022 11:43, Christian König wrote:
> 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.

Cool, lets see about the specifics below and then decide.
  
> 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.

By backend you mean drivers/dma-buf/dma-fence.c and by front end driver specific implementations? Or simply users for dma-fence?

Could be that I got confused.. I was reading these two:


	 * This callback is optional. If this callback is not present, then the
	 * driver must always have signaling enabled.
	 */
	bool (*enable_signaling)(struct dma_fence *fence);

And dma_fence_is_signaled:

  * Returns true if the fence was already signaled, false if not. Since this
  * function doesn't enable signaling, it is not guaranteed to ever return
  * true if dma_fence_add_callback(), dma_fence_wait() or
  * dma_fence_enable_sw_signaling() haven't been called before.

Right, I think I did get confused, apologies. What I was thinking was probably two separate conditions:

  static inline bool
  dma_fence_is_signaled(struct dma_fence *fence)
  {
+#ifdef CONFIG_DEBUG_DMAFENCE
+       if (WARN_ON_ONCE(!fence->ops->enable_signaling &&
+                        !test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags)))
+               return false;
+
+       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;

Not sure "is signaled" is the best place for the first one or that it should definitely be added.

Regards,

Tvrtko

> 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 at amd.com>
>>>
>>> With the improved commit message this patch is Reviewed-by: Christian 
>>> König <christian.koenig at amd.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;
>>>
>