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

From: Al Viro
Date: Mon Mar 18 2013 - 19:01:19 EST

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

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...
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