Messages in this thread | | | Date | Mon, 18 Mar 2013 13:31:55 +1100 | From | Dave Chinner <> | Subject | Re: [PATCH 2/9] vfs: export do_splice_direct() to modules |
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On Sun, Mar 17, 2013 at 01:06:59PM +0000, David Howells wrote: > Miklos Szeredi <miklos@szeredi.hu> wrote: > > > Export do_splice_direct() to modules. Needed by overlay filesystem. > > Apparently you cannot call this from any function that is holding an i_mutex > if the target of the splice uses generic_file_splice_write(). > > The problem is a potential deadlock situation: > > We have places already that do: > > mnt_want_write() > mutex_lock() > > This can be found in do_last() for example. > > However, mnt_want_write() calls sb_start_write() as does > generic_file_splice_write(). So now in ovl_copy_up_locked() you're adding: > > mutex_lock() > sb_start_write() > > which lockdep reports as a potential ABBA deadlock. > > Now, looking at __sb_start_write(), I'm not entirely sure how the deadlock > might operate, so it's possible that this is a false alarm. Maybe Jan Kara can > illuminate further, so I've added him to the cc list. > > I've attached the report I got with unionmount.
There's plenty of problems with splice locking that can lead to deadlocks. Here's another that's been known for ages:
http://oss.sgi.com/archives/xfs/2011-08/msg00168.html
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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