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

[Christian König]

Am 06.09.22 um 12:20 schrieb Tvrtko Ursulin:
> 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?

Yeah, I was thinking about that as well.

The slowpath config option was just at hand because when you want to 
test the slowpath you want to test this here as well.

> 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.
> +        */

Well that comment is a bit misleading. The intention of the extra check 
is to find bugs in the frontend and not the backend.

In other words somewhere in the drm_syncobj code we have a 
dma_fence_is_signaled() call without matching 
dma_fence_enable_sw_signaling().

Regards,
Christian.

> + 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@xxxxxxx>
> >
> > With the improved commit message this patch is Reviewed-by: Christian 
> > König <christian.koenig@xxxxxxx>
> >
> > 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;