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    Subject[PATCH] writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue
    On Thu, Jan 19, 2012 at 01:39:21AM +0530, Rabin Vincent wrote:
    > On Tue, Jan 17, 2012 at 09:02, Wu Fengguang <fengguang.wu@intel.com> wrote:
    > > On Sun, Jan 15, 2012 at 08:58:06PM +0530, Rabin Vincent wrote:
    > >>  Unable to handle kernel NULL pointer dereference at virtual address 0000002c
    > >>  pgd = c0004000
    > >>  [0000002c] *pgd=00000000
    > >>  Internal error: Oops: 17 [#1] PREEMPT SMP
    > >>  PC is at ftrace_raw_event_writeback_single_inode_template+0x60/0xe4
    > >>  LR is at ftrace_raw_event_writeback_single_inode_template+0x50/0xe4
    > >>
    > >> The full trace+log is attached.  My kernel (current linus) has a delay
    > >> inserted in __mark_inode_dirty, to easily trigger the condition:
    > >
    > > Rabin, thanks for showing the helpful details! It should be fixable by
    > > the use of inode_to_bdi():
    >
    > Thanks, this fixes that one.
    >
    > However, I've found one more race condition leading to a crash when
    > tracing is enabled, this time from the writeback:queue trace point from
    > bdi_queue_work(). The cause is the same, i.e. bdi->dev is NULL. This
    > was produced with the help of the following delay patch. trace+log is
    > attached.

    Rabin, this should fix the bug. Note that I take no efforts to remove
    the to-be-queued and already-queued works. I'm also a bit afraid if
    the traces in the balance_dirty_pages() path (trace_balance_dirty_pages,
    trace_bdi_dirty_ratelimit and writeback_wake_background) will have
    similar NULL dereference bug. Do you test it by physically hot
    removing a SD card, or with some detach command or emulation?

    Thanks,
    Fengguang

    ---
    Subject: writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue
    Date: Sat Feb 04 20:54:03 CST 2012

    When the SD card is hot removed without umount, del_gendisk() will call
    bdi_unregister() but not destroy/free it. This leaves the bdi in the
    bdi->dev = NULL, bdi->wb.task = NULL, bdi->bdi_list removed state.

    If someone 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>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    include/trace/events/writeback.h | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    --- linux-next.orig/include/trace/events/writeback.h 2012-02-04 20:51:01.000000000 +0800
    +++ linux-next/include/trace/events/writeback.h 2012-02-04 20:54:00.000000000 +0800
    @@ -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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