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    Subject[16/28] writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue
    3.2-stable review patch.  If anyone has any objections, please let me know.

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

    From: Wu Fengguang <fengguang.wu@intel.com>

    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@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>

    ---
    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
    @@ -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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