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

Christian König
2022-09-05T18:26:54Z

Am 05.09.22 um 18:39 schrieb Tvrtko Ursulin:
>
> 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@amd.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?

We have a bug that the drm_sync file doesn't properly enable signaling
leading to an igt test failure.

>
> 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?

That's neither an error nor impossible. For debugging we just never
return signaled from the dma_fence_is_signaled() function when signaling
was not enabled before.

I also plan to remove the return value from the enable_signaling
callback. That was just not very well designed.

>
> 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?

Oh, good point. That sounds like we have some bug in the core dma_fence
code as well.

Calls to dma_fence_add_callback() and dma_fence_wait() should enable
signaling implicitly and don't need an extra call for that.

Only dma_fence_is_signaled() needs this explicit enabling of signaling
through dma_fence_enable_sw_signaling().

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

I think the kerneldoc is just a bit misleading. The point is that when
you need to call dma_fence_enable_sw_signaling() you must hold a
reference to the fence object.

But that's true for all the dma_fence_* functions. The race described in
the comment is just nonsense because you need to hold that reference anyway.

Regards,
Christian.

>
> 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;
>>