Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

From: J. R. Okajima
Date: Fri Mar 22 2013 - 13:38:07 EST

David Howells:
> 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.

It is related to the design of UnionMount, isn't it?
UnionMount is not a filesystem and doen't have its own superblock.
If it was a fs, then
- vfs_truncate() acquires sb_writers for the unioning-fs.
- the unioning-fs may call vfs_truncate() again for the underlying fs.
- this time, sb_writers is for the underlying fs which is a different
sb_writers object from the already acquired one.
So there would be no deadlock.

Still lockdep will produce the message since sb_writers doesn't have the
lockdep class. Of course, we can introduce the lock class for it, or call
lockdep_off()/on() simply in order to stop the message. But, as long as
the unioning feature is not implemented as a fs, the solution will not
be so easy. I am afraid UnionMount will need to introduce a new counter
(or a new flag) to indicate the task entered the union, and adjust the
lock class or decide to call lockdep_off() for sb_writers. I don't think
it is a good idea.

J. R. Okajima
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