linux-kernel - Re: [PATCH] debugfs: fix potential infinite loop in debugfs_remove

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Date:   Thu, 14 Nov 2019 09:34:10 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     <gregkh@...uxfoundation.org>, <rafael@...nel.org>,
        <oleg@...hat.com>, <jack@...e.cz>, <linux-kernel@...r.kernel.org>,
        <zhengbin13@...wei.com>, <yi.zhang@...wei.com>,
        <chenxiang66@...ilicon.com>, <xiexiuqi@...wei.com>,
        Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] debugfs: fix potential infinite loop in
 debugfs_remove_recursive

On Thu, 14 Nov 2019 14:59:04 +0800
"yukuai (C)" <yukuai3@...wei.com> wrote:

> > Have you tried this patch with lockdep enabled and tried to hit this
> > code path?
> > 

> >   
> You are right, I get the results with lockdep enabled:

That was what I was afraid of :-(

> [   64.314748] ============================================
> [   64.315568] WARNING: possible recursive locking detected
> [   64.316549] 5.4.0-rc7-dirty #5 Tainted: G           O
> [   64.317398] --------------------------------------------
> [   64.318230] rmmod/2607 is trying to acquire lock:

> 
> The warning will disappeare by adding 
> lockdep_set_novalidate_class(&child->d_lock) before calling 
> simple_empty(child). But I'm not sure It's the right modfication.

I'm wondering if we should add a simple_empty_unlocked() that does
simple_empty() without taking the lock, to allow us to call
spin_lock_nested() on the child. Of course, I don't know how much
nesting we allow as it calls the nesting too.

This looks to be something that the vfs folks need to look at.

-- Steve