In dma_fence_release() there is a WARN_ON which could be triggered by
several cases of wrong dma-fence usage. This patch adds a comment to
explain two use-cases to help driver developers that use dma-fence
and trigger that WARN_ON to better understand the reasons for it.
Signed-off-by: Oded Gabbay <oded.gabbay@xxxxxxxxx>
---
drivers/dma-buf/dma-fence.c | 33 +++++++++++++++++++++++++++++++++
1 file changed, 33 insertions(+)
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 5d101c4053e0..a7170ab23ec0 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -171,6 +171,39 @@ void dma_fence_release(struct kref *kref)
trace_dma_fence_destroy(fence);
+ /*
+ * If the WARN_ON below is triggered it could be because the dma fence
+ * was not signaled and therefore, the cb list is still not empty
+ * because the cb functions were not called.
+ *
+ * A more subtle case is where the fence got signaled by a thread that
+ * didn't hold a ref to the fence. The following describes the scenario:
+ *
+ * Thread A Thread B
+ *-------------------------- --------------------------
+ * calls dma_fence_signal() {
+ * set signal bit
+ *
+ * scheduled out
+ * ---------------------------> calls dma_fence_wait_timeout() and
+ * returns immediately
+ *
+ * calls dma_fence_put()
+ * |
+ * |thread A doesn't hold ref
+ * |to fence so ref goes to 0
+ * |and release is called
+ * |
+ * -> dma_fence_release()
+ * |
+ * -> WARN_ON triggered
+ *
+ * go over CB list,
+ * call each CB and remove it
+ * }
+ *
+ *
+ */
WARN_ON(!list_empty(&fence->cb_list));
if (fence->ops->release)
--
2.14.3
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