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In gst_v4l2_allocator_qbuf(), the memory is referenced after the buffer is queued. Once queued (VIDIOC_QBUF), the buffer might be handled by the V4L2 driver (e.g. decoded) and dequeued (gst_v4l2_allocator_dqbuf), through a different thread, before the memory is referenced (gst_memory_ref). In this case, in gst_v4l2_allocator_dqbuf(), the memory is unreferenced (gst_memory_unref) before having been referenced: the memory refcount reaches 0, and the memory is freed. So, to avoid this crossing case, in gst_v4l2_allocator_qbuf(), the memory shall be referenced before the buffer is queued. https://bugzilla.gnome.org/show_bug.cgi?id=777399
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