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

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Message-ID: <ec41b299-4280-d8e4-7ab0-23b5ea6ad401@linux.intel.com>
Date:   Tue, 6 Sep 2022 11:20:52 +0100
From:   Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:     Christian König <christian.koenig@....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


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?

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.
+        */
+       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;
>