linux-kernel - Re: [PATCH 1/4] dma-buf: Check status of enable-signaling bit on debug

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Message-ID: <3c702549-75f4-c640-9f9c-37d7fcbb1645@linux.intel.com>
Date:   Mon, 5 Sep 2022 17:39:39 +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 1/4] dma-buf: Check status of enable-signaling bit on
 debug


On 05/09/2022 12:21, Christian König wrote:
> Am 05.09.22 um 12:56 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.
>> 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.
> 
> You might want to put this patch at the end of the series to avoid 
> breaking the kernel in between.
> 
>>
>> Signed-off-by: Arvind Yadav <Arvind.Yadav@....com>
>> ---
>>   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..60c0e935c0b5 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_FS
> 
> CONFIG_DEBUG_FS is certainly wrong, probably better to check for 
> CONFIG_DEBUG_WW_MUTEX_SLOWPATH here.
> 
> Apart from that looks good to me,

What's the full story in this series - I'm afraid the cover letter does not make it clear to a casual reader like myself? Where does the difference between debug and non debug kernel come from?

And how do the proposed changes relate to the following kerneldoc excerpt:

	 * Since many implementations can call dma_fence_signal() even when before
	 * @enable_signaling has been called there's a race window, where the
	 * dma_fence_signal() might result in the final fence reference being
	 * released and its memory freed. To avoid this, implementations of this
	 * callback should grab their own reference using dma_fence_get(), to be
	 * released when the fence is signalled (through e.g. the interrupt
	 * handler).
	 *
	 * This callback is optional. If this callback is not present, then the
	 * driver must always have signaling enabled.

Is it now an error, or should be impossible condition, for "is signaled" to return true _unless_ signaling has been enabled?

If the statement (in a later patch) is signalling should always be explicitly enabled by the callers of dma_fence_add_callback, then what about the existing call to __dma_fence_enable_signaling from dma_fence_add_callback?

Or if the rules are changing shouldn't kerneldoc be updated as part of the series?

Regards,

Tvrtko

> Christian.
> 
>> +    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;
>