Re: [PATCH] bdi: Fix oops in wb_workfn()

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On Thu 03-05-18 18:26:26, Jan Kara wrote:
> Syzbot has reported that it can hit a NULL pointer dereference in
> wb_workfn() due to wb->bdi->dev being NULL. This indicates that
> wb_workfn() was called for an already unregistered bdi which should not
> happen as wb_shutdown() called from bdi_unregister() should make sure
> all pending writeback works are completed before bdi is unregistered.
> Except that wb_workfn() itself can requeue the work with:
> 
> 	mod_delayed_work(bdi_wq, &wb->dwork, 0);
> 
> and if this happens while wb_shutdown() is waiting in:
> 
> 	flush_delayed_work(&wb->dwork);
> 
> the dwork can get executed after wb_shutdown() has finished and
> bdi_unregister() has cleared wb->bdi->dev.
> 
> Make wb_workfn() use wakeup_wb() for requeueing the work which takes all
> the necessary precautions against racing with bdi unregistration.
> 
> CC: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> CC: Tejun Heo <tj@xxxxxxxxxx>
> Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977
> Reported-by: syzbot <syzbot+9873874c735f2892e7e9@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
>  fs/fs-writeback.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Jens, can you please pick up this patch? Probably for the next merge window
(I don't see a reason to rush this at this point in release cycle). Thanks!

								Honza

> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 47d7c151fcba..471d863958bc 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1961,7 +1961,7 @@ void wb_workfn(struct work_struct *work)
>  	}
>  
>  	if (!list_empty(&wb->work_list))
> -		mod_delayed_work(bdi_wq, &wb->dwork, 0);
> +		wb_wakeup(wb);
>  	else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
>  		wb_wakeup_delayed(wb);
>  
> -- 
> 2.13.6
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR



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