Re: [Intel-gfx] [PATCH 1/2] dma-fence: Serialise signal enabling (dma_fence_enable_sw_signaling)

[Tvrtko Ursulin]

On 15/09/2019 19:45, Chris Wilson wrote:

Make dma_fence_enable_sw_signaling() behave like its
dma_fence_add_callback() and dma_fence_default_wait() counterparts and
perform the test to enable signaling under the fence->lock, along with
the action to do so. This ensure that should an implementation be trying
to flush the cb_list (by signaling) on retirement before freeing the
fence, it can do so in a race-free manner.

See also 0fc89b6802ba ("dma-fence: Simply wrap dma_fence_signal_locked
with dma_fence_signal").

Signed-off-by: Chris Wilson 
---
  drivers/dma-buf/dma-fence.c | 11 +--
  1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 2c136aee3e79..587727089134 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -285,19 +285,18 @@ void dma_fence_enable_sw_signaling(struct dma_fence 
*fence)
  {
unsigned long flags;
  
+	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))

+   return;
+
+   spin_lock_irqsave(fence->lock, flags);
if (!test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,


I dare not to ask if this couldn't be the non-atomic version, but I have 
empirically proven to myself things are not that straightforward around 
here.



  &fence->flags) &&
-   !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&
fence->ops->enable_signaling) {
trace_dma_fence_enable_signal(fence);
-
-   spin_lock_irqsave(fence->lock, flags);
-
if (!fence->ops->enable_signaling(fence))
dma_fence_signal_locked(fence);
-
-   spin_unlock_irqrestore(fence->lock, flags);
}
+   spin_unlock_irqrestore(fence->lock, flags);
  }
  EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
  



Reviewed-by: Tvrtko Ursulin 

Regards,

Tvrtko
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[Intel-gfx] [PATCH 1/2] dma-fence: Serialise signal enabling (dma_fence_enable_sw_signaling)

[Chris Wilson]

Make dma_fence_enable_sw_signaling() behave like its
dma_fence_add_callback() and dma_fence_default_wait() counterparts and
perform the test to enable signaling under the fence->lock, along with
the action to do so. This ensure that should an implementation be trying
to flush the cb_list (by signaling) on retirement before freeing the
fence, it can do so in a race-free manner.

See also 0fc89b6802ba ("dma-fence: Simply wrap dma_fence_signal_locked
with dma_fence_signal").

Signed-off-by: Chris Wilson 
---
 drivers/dma-buf/dma-fence.c | 11 +--
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 2c136aee3e79..587727089134 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -285,19 +285,18 @@ void dma_fence_enable_sw_signaling(struct dma_fence 
*fence)
 {
unsigned long flags;
 
+   if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
+   return;
+
+   spin_lock_irqsave(fence->lock, flags);
if (!test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
  &fence->flags) &&
-   !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&
fence->ops->enable_signaling) {
trace_dma_fence_enable_signal(fence);
-
-   spin_lock_irqsave(fence->lock, flags);
-
if (!fence->ops->enable_signaling(fence))
dma_fence_signal_locked(fence);
-
-   spin_unlock_irqrestore(fence->lock, flags);
}
+   spin_unlock_irqrestore(fence->lock, flags);
 }
 EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
 
-- 
2.23.0

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