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writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue

commit 977b7e3 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@rab.in>
Tested-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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1 parent aec14f4 commit 422204b77968331ce85721527f7ece49f72658f2 @fengguang fengguang committed with gregkh Feb 5, 2012
Showing with 4 additions and 1 deletion.
  1. +4 −1 include/trace/events/writeback.h
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5 include/trace/events/writeback.h
@@ -47,7 +47,10 @@ DECLARE_EVENT_CLASS(writeback_work_class,
__field(int, reason)
),
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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