dma-fence: Serialise signal enabling (dma_fence_enable_sw_signaling)

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 0fc89b6802 ("dma-fence: Simply wrap dma_fence_signal_locked
with dma_fence_signal").

v2: Refactor all 3 enable_signaling paths to use a common function.
v3: Don't argue, just keep the tracepoint in the existing spot.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004101140.32713-1-chris@chris-wilson.co.uk

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