linux-kernel - Re: fs: lockup on rename

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Message-ID: <20141026234230.GU7996@ZenIV.linux.org.uk>
Date:	Sun, 26 Oct 2014 23:42:30 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: fs: lockup on rename_mutex in fs/dcache.c:1035

On Sun, Oct 26, 2014 at 04:33:11PM -0700, Linus Torvalds wrote:
> On Sun, Oct 26, 2014 at 2:57 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >  .. snip ..
> > in d_walk(), __list_del() instead of list_del() in __dentry_kill(), d_u.d_child
> > turning into d_child everywhere, while d_alias turns into d_u.d_alias...
> >
> > It looks like that way we would get no retries on the second pass.  Comments?
> 
> Since I missed the whole issue with d_child.next, I'm not sure any
> comments from me would be helpful.
> 
> It does sound like trying to be more careful with d_child and using
> d_alias instead is a good idea. d_alias is only used under the dentry
> lock, and not in any horribly subtle situations, right? So that sounds
> like a good change regardless of this particular fix - making the
> union happen in a less nasty place..

OK...  See vfs.git#experimental-d_walk - the first commit there moves
d_rcu from d_child to d_alias, the second is basically "skip the killed
siblings rather than restarting the tree walk".
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