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

[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@xxxxxxx>
> > ---
> >   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;