linux-kernel - Re: [overlayfs] lockdep splat after mounting overlayfs over overlayfs

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Date:	Thu, 25 Jun 2015 23:28:20 -0400 (EDT)
From:	Xu Wang <xuw@...hat.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-unionfs@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [overlayfs] lockdep splat after mounting overlayfs over
 overlayfs

> On Thu, Jun 25, 2015 at 10:24 AM, Xu Wang <xuw@...hat.com> wrote:
> >> I've accidentally mounted one overlayfs over another and got obvious
> >> warning from lockdep: i_mutex lockdep classes are per-fs-type.
> >>
> >> # mount -t overlay overlay 1 -o
> >> upperdir=1_upper,workdir=1_work,lowerdir=1_lower
> >> # mount -t overlay overlay 2 -o upperdir=2_upper,workdir=2_work,lowerdir=1
> >> # ls 2
> >
> > This reporting is positive, we are not in deadlock situation actually. The
> > "2" dir
> > of overlayfs will call iterate_dir->"1" dir of overlayfs->iterate_dir, and
> > the nest
> > iterate_dir happened on the same file system, so the warning came out.
> >
> > We'd better make the lower and upper in different fs instance, and this
> > warning
> > will disappear.
> >
> > And this lockdep warning happened when the nest iterate_dir call of same
> > fs(I
> > mean the same super block). The function check_deadlock in lockdep.c will
> > report the nest lock of same process. If we make the 2_upper and 2_work in
> > a different fs, no warning comes out.
> 
> Yep, it's not a deadlock. As I mentioned lockdep classes are per-fs-type so
> lockdep cannot see difference between i_mutexes on different sb of the
> same type.
> But anyway this looks messy.
>
 
yes, you are right. The i_mutex_class is file_system_tye scale. I was puzzled by
the debug_locks mechanism during my quick tests. The nest iterate_dir is overlay
dir, neither upper nor lower.

> Probably it's safer to forbid overlayfs as lower or upper mount for overlayfs
> because this have no sense. Nesting anyway is limited by the depth of kernel
> stack and sb->s_stack_depth.
> 
> Or overlayfs could detect this situation and substitute layers of underlying
> overlayfs into its own lower layers in appropriate place.
> 

Can we add the lockdep_off/lockdep_on in this situation? For we know this is
just the false positive reporting of lockdep.


Thanks,

Xu Wang
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