Re: [syzbot] [gfs2?] WARNING: suspicious RCU usage in gfs2_permission

[Al Viro]

On Fri, Oct 20, 2023 at 12:10:38AM -0700, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit 0abd1557e21c617bd13fc18f7725fc6363c05913
> Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Date:   Mon Oct 2 02:33:44 2023 +0000
> 
>     gfs2: fix an oops in gfs2_permission
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=10b21c33680000
> start commit:   2dac75696c6d Add linux-next specific files for 20231018
> git tree:       linux-next
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=12b21c33680000
> console output: https://syzkaller.appspot.com/x/log.txt?x=14b21c33680000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=6f8545e1ef7a2b66
> dashboard link: https://syzkaller.appspot.com/bug?extid=3e5130844b0c0e2b4948
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=101c8d09680000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11a07475680000
> 
> Reported-by: syzbot+3e5130844b0c0e2b4948@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 0abd1557e21c ("gfs2: fix an oops in gfs2_permission")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Complaints about rcu_dereference() outside of rcu_read_lock().

We could replace that line with
	if (mask & MAY_NOT_BLOCK)
		gl = rcu_dereference(ip->i_gl);
	else
		gl = ip->i_gl;
or by any equivalent way to tell lockdep it ought to STFU.
BTW, the amount of rcu_dereference_protected(..., true) is somewhat depressing...

Probably need to turn
                ip->i_gl = NULL;
in the end of gfs2_evict_inode() into rcu_assign_pointer(ip->i_gl, NULL);
and transpose it with the previous line -
                gfs2_glock_put_eventually(ip->i_gl);

I don't think it really matters in this case, though - destruction of the object
it used to point to is delayed in all cases.  Matter of taste (and lockdep
false positives)...