Re: [PATCH AUTOSEL 4.19 11/12] gfs2: Refcounting fix in gfs2_thaw_super

[Sasha Levin]

On Thu, Jan 18, 2024 at 12:50:37PM +0100, Andreas Gruenbacher wrote:
> On Tue, Jan 16, 2024 at 9:53 PM Pavel Machek <pavel@xxxxxxx> wrote:
> > Hi!
> >
> > > From: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
> > >
> > > [ Upstream commit 4e58543e7da4859c4ba61d15493e3522b6ad71fd ]
> > >
> > > It turns out that the .freeze_super and .thaw_super operations require
> > > the filesystem to manage the superblock refcount itself.  We are using
> > > the freeze_super() and thaw_super() helpers to mostly take care of that
> > > for us, but this means that the superblock may no longer be around by
> > > when thaw_super() returns, and gfs2_thaw_super() will then access freed
> > > memory.  Take an extra superblock reference in gfs2_thaw_super() to fix
> > > that.
> >
> > Patch was broken during backport.
> >
> > > +++ b/fs/gfs2/super.c
> > > @@ -1013,6 +1013,7 @@ static int gfs2_freeze(struct super_block *sb)
> > >               goto out;
> > >       }
> > >
> > > +     atomic_inc(&sb->s_active);
> > >       for (;;) {
> > >               error = gfs2_lock_fs_check_clean(sdp, &sdp->sd_freeze_gh);
> > >               if (!error)
> > > @@ -1034,6 +1035,7 @@ static int gfs2_freeze(struct super_block *sb)
> > >       error = 0;
> > >  out:
> > >       mutex_unlock(&sdp->sd_freeze_mutex);
> > > +     deactivate_super(sb);
> > >       return error;
> > >  }
> >
> > Notice the goto out? That now jumps around the atomic_inc, but we
> > still do decrease. This will break 4.19, please fix or drop.
>
> Thanks, Pavel.
>
> Sasha, you don't want that fix without "gfs2: Rework freeze / thaw
> logic" and the follow-up fixes, and backporting that probably isn't
> going to be worth it.

I'll drop it, thanks!

--
Thanks,
Sasha