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

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Message-ID: <f2e1367f-b056-b2af-365c-8ae4ef03f008@amd.com>
Date:   Tue, 6 Sep 2022 10:39:53 +0200
From:   Christian König <christian.koenig@....com>
To:     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 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.
"

>
> 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;
>