>On Mon, Jan 02, 2012 at 06:38:21PM +0900, ����ȣ wrote:
>> from Chanho Min <chanho.min@xxxxxxx>
>>
>> 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.
>
It is not race condition. This happens when USB is removed during write-access.
bdi_wakeup_thread_delayed is called after kthread_stop, and timer is activated again.
bdi_unregister
kthread_stop
bdi_wakeup_thread_delayed (sys_write mostly calls this)
timer fires
Anyway,Is this safeguard to prevent from waking up killed thread?
Thanks,
Chanho
>Thanks,
>Fengguang
>
>
>> Signed-off-by: Chanho Min <chanho.min@xxxxxxx>
>> ---
>> 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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