linux-kernel - Re: [PATCH 2/9] vfs: export do_splice

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Date:	Tue, 19 Mar 2013 21:54:03 +0100
From:	Jan Kara <jack@...e.cz>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Jan Kara <jack@...e.cz>, David Howells <dhowells@...hat.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	akpm@...ux-foundation.org, apw@...onical.com, nbd@...nwrt.org,
	neilb@...e.de, jordipujolp@...il.com, ezk@....cs.sunysb.edu,
	sedat.dilek@...glemail.com, hooanon05@...oo.co.jp, mszeredi@...e.cz
Subject: Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

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:
> > > 
> > >   THREAD1		THREAD2				THREAD3
> > > 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.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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