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

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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;




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