[08/15] writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue

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3.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Wu Fengguang <fengguang.wu@xxxxxxxxx>

commit 977b7e3a52a7421ad33a393a38ece59f3d41c2fa upstream.

When a SD card is hot removed without umount, del_gendisk() will call
bdi_unregister() without destroying/freeing it. This leaves the bdi in
the bdi->dev = NULL, bdi->wb.task = NULL, bdi->bdi_list removed state.

When sync(2) gets the bdi before bdi_unregister() and calls
bdi_queue_work() after the unregister, trace_writeback_queue will be
dereferencing the NULL bdi->dev. Fix it with a simple test for NULL.

LKML-reference: http://lkml.org/lkml/2012/1/18/346
Reported-by: Rabin Vincent <rabin@xxxxxx>
Tested-by: Namjae Jeon <linkinjeon@xxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/trace/events/writeback.h |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -23,7 +23,10 @@ DECLARE_EVENT_CLASS(writeback_work_class
 		__field(int, for_background)
 	),
 	TP_fast_assign(
-		strncpy(__entry->name, dev_name(bdi->dev), 32);
+		struct device *dev = bdi->dev;
+		if (!dev)
+			dev = default_backing_dev_info.dev;
+		strncpy(__entry->name, dev_name(dev), 32);
 		__entry->nr_pages = work->nr_pages;
 		__entry->sb_dev = work->sb ? work->sb->s_dev : 0;
 		__entry->sync_mode = work->sync_mode;


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