Re: [PATCH 2/5] dma-fence: Serialise signal enabling (dma_fence_enable_sw_signaling)

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On 03/10/2019 22:00, 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").

v2: Refactor all 3 enable_signaling paths to use a common function.

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
  drivers/dma-buf/dma-fence.c | 78 +++++++++++++++++--------------------
  1 file changed, 35 insertions(+), 43 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 2c136aee3e79..b58528c1cc9d 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -273,6 +273,30 @@ void dma_fence_free(struct dma_fence *fence)
  }
  EXPORT_SYMBOL(dma_fence_free);
+static bool __dma_fence_enable_signaling(struct dma_fence *fence)
+{
+	bool was_set;
+
+	lockdep_assert_held(fence->lock);
+
+	was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
+				   &fence->flags);
+
+	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
+		return false;
+
+	if (!was_set && fence->ops->enable_signaling) {
+		if (!fence->ops->enable_signaling(fence)) {
+			dma_fence_signal_locked(fence);
+			return false;
+		}
+
+		trace_dma_fence_enable_signal(fence);

Tracepoint used to come before the enable_signaling call so probably best to keep it like that.

+	}
+
+	return true;
+}
+
  /**
   * dma_fence_enable_sw_signaling - enable signaling on fence
   * @fence: the fence to enable
@@ -285,19 +309,12 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence)
  {
  	unsigned long 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);
+	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
+		return;
- spin_unlock_irqrestore(fence->lock, flags);
-	}
+	spin_lock_irqsave(fence->lock, flags);
+	__dma_fence_enable_signaling(fence);
+	spin_unlock_irqrestore(fence->lock, flags);
  }
  EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
@@ -331,7 +348,6 @@ int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
  {
  	unsigned long flags;
  	int ret = 0;
-	bool was_set;
if (WARN_ON(!fence || !func))
  		return -EINVAL;
@@ -343,25 +359,14 @@ int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
spin_lock_irqsave(fence->lock, flags); - was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
-				   &fence->flags);
-
-	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
-		ret = -ENOENT;
-	else if (!was_set && fence->ops->enable_signaling) {
-		trace_dma_fence_enable_signal(fence);
-
-		if (!fence->ops->enable_signaling(fence)) {
-			dma_fence_signal_locked(fence);
-			ret = -ENOENT;
-		}
-	}
-
-	if (!ret) {
+	if (__dma_fence_enable_signaling(fence)) {
  		cb->func = func;
  		list_add_tail(&cb->node, &fence->cb_list);
-	} else
+	} else {
  		INIT_LIST_HEAD(&cb->node);
+		ret = -ENOENT;
+	}
+
  	spin_unlock_irqrestore(fence->lock, flags);
return ret;
@@ -461,7 +466,6 @@ dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout)
  	struct default_wait_cb cb;
  	unsigned long flags;
  	signed long ret = timeout ? timeout : 1;
-	bool was_set;
if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
  		return ret;
@@ -473,21 +477,9 @@ dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout)
  		goto out;
  	}
- was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
-				   &fence->flags);
-
-	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
+	if (!__dma_fence_enable_signaling(fence))
  		goto out;
- if (!was_set && fence->ops->enable_signaling) {
-		trace_dma_fence_enable_signal(fence);
-
-		if (!fence->ops->enable_signaling(fence)) {
-			dma_fence_signal_locked(fence);
-			goto out;
-		}
-	}
-
  	if (!timeout) {
  		ret = 0;
  		goto out;


The rest looks correct to me.

Regards,

Tvrtko
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