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

From: Jan Kara
Date: Tue Mar 19 2013 - 16:54:18 EST

On Mon 18-03-13 23:01:03, Al Viro wrote:
> On Mon, Mar 18, 2013 at 09:53:34PM +0000, Al Viro wrote:
> > On Mon, Mar 18, 2013 at 04:39:36PM +0100, Jan Kara wrote:
> > > IMO the deadlock is real. In freeze_super() we wait for all writers to
> > > the filesystem to finish while blocking beginning of any further writes. So
> > > we have a deadlock scenario like:
> > >
> > > mnt_want_write() mutex_lock(&inode->i_mutex);
> > > ... freeze_super()
> > > block on mutex_lock(&inode->i_mutex)
> > > sb_wait_write(sb, SB_FREEZE_WRITE);
> > > block in sb_start_write()
> >
> > The bug is on fsfreeze side and this is not the only problem related to it.
> > I've missed the implications when I applied "fs: Add freezing handling
> > to mnt_want_write() / mnt_drop_write()" last June ;-/
> >
> > The thing is, until then mnt_want_write() used to be a counter; it could be
> > nested. Now any such nesting is a deadlock you've just described. This
> > is seriously wrong, IMO.
> >
> > BTW, having sb_start_write() buried in individual ->splice_write() is
> > asking for trouble; could you describe the rules for that? E.g. where
> > does it nest wrt filesystem-private locks? XFS iolock, for example...
> I'm looking at the existing callers and I really wonder if we ought to
> push sb_start_write() from ->splice_write()/->aio_write()/etc. into the
> callers.
Yeah, that should be OK.

> Something like file_start_write()/file_end_write(), with check for file
> being regular one might be a good starting point. As it is, copyup is
> really fucked both in unionmount and overlayfs...
Makes sense. I can do the patch. BTW, for months I'm trying to push to you
a patch which creates a function like file_start_write() which returns
EAGAIN if the file is open with O_NONBLOCK and fs is frozen (this allows me
to solve a deadlock with bsd process accounting to frozen fs). After this
change the patch will become trivial so I'll add it to the series and
hopefully it won't get ignored.

Jan Kara <jack@xxxxxxx>
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