linux-kernel - Re: [PATCH AUTOSEL 4.19 11/12] gfs2: Refcounting fix in gfs2_thaw

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Date: Thu, 18 Jan 2024 12:50:37 +0100
From: Andreas Gruenbacher <agruenba@...hat.com>
To: Sasha Levin <sashal@...nel.org>, Pavel Machek <pavel@...x.de>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org, gfs2@...ts.linux.dev
Subject: Re: [PATCH AUTOSEL 4.19 11/12] gfs2: Refcounting fix in gfs2_thaw_super

On Tue, Jan 16, 2024 at 9:53 PM Pavel Machek <pavel@...x.de> wrote:
> Hi!
>
> > From: Andreas Gruenbacher <agruenba@...hat.com>
> >
> > [ 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.

Thanks,
Andreas