Re: [PATCH] mm/backing-dev.c: fix crash when USB/SCSI device isdetached

[Wu Fengguang]

On Tue, Jan 03, 2012 at 12:23:44PM +0900, Chanho Min wrote:
> >On Mon, Jan 02, 2012 at 06:38:21PM +0900, ïïïïÈ 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

Ah OK, the timer could be restarted in the mean while, which breaks
the synchronization rule in del_timer_sync().

I noticed a related fix is merged recently, does your test kernel
contain this commit?

commit 7a401a972df8e184b3d1a3fc958c0a4ddee8d312
Author: Rabin Vincent <rabin.vincent@xxxxxxxxxxxxxx>
Date:   Fri Nov 11 13:29:04 2011 +0100

    backing-dev: ensure wakeup_timer is deleted

> Anyway,Is this safeguard to prevent from waking up killed thread?

This patch makes no guarantee wakeup_timer_fn() will see NULL
bdi->wb.task before the task is stopped, so there is still race
conditions. And still, the complete fix would be to prevent
wakeup_timer_fn() from being called at all.

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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