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    SubjectRe: [PATCH] mm/backing-dev.c: fix crash when USB/SCSI device is detached
    On Mon, Jan 02, 2012 at 06:38:21PM +0900, ����ȣ wrote:
    > from Chanho Min <chanho.min@lge.com>
    >
    > System may crash in backing-dev.c when removal SCSI device is detached.
    > bdi task is killed by bdi_unregister()/'khubd', but task's point remains.
    > Shortly afterward, If 'wb->wakeup_timer' is expired before
    > del_timer()/bdi_forker_thread,
    > wakeup_timer_fn() may wake up the dead thread which cause the crash.
    > 'bdi->wb.task' should be NULL as this patch.

    Is it some race condition between del_timer() and del_timer_sync()?

    bdi_unregister() calls

    del_timer_sync
    bdi_wb_shutdown
    kthread_stop

    in turn, and del_timer_sync() should guarantee wakeup_timer_fn() is
    no longer called to access the stopped task.

    Thanks,
    Fengguang


    > Signed-off-by: Chanho Min <chanho.min@lge.com>
    > ---
    > mm/backing-dev.c | 1 +
    > 1 files changed, 1 insertions(+), 0 deletions(-)
    >
    > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
    > index 71034f4..4378a5e 100644
    > --- a/mm/backing-dev.c
    > +++ b/mm/backing-dev.c
    > @@ -607,6 +607,7 @@ static void bdi_wb_shutdown(struct backing_dev_info
    > *bdi)
    > if (bdi->wb.task) {
    > thaw_process(bdi->wb.task);
    > kthread_stop(bdi->wb.task);
    > + bdi->wb.task = NULL;
    > }
    > }
    >
    > --
    > 1.7.0.4
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