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

[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?

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.
+        */
+       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;