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

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Date:   Thu, 14 Nov 2019 20:53:11 -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 Fri, 15 Nov 2019 09:47:38 +0800
"yukuai (C)" <yukuai3@...wei.com> wrote:

> On 2019/11/14 22:34, Steven Rostedt wrote:
> > 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.  
> Do you think we can do this:
> 1. add a new enum type for dentry_d_lock_class:
> enum dentry_d_lock_class
> {
> 	DENTRY_D_LOCK_NORMAL, /* implicitly used by plain spin_lock() APIs. */
> 	DENTRY_D_LOCK_NESTED
> 	DENTRY_D_LOCK_NESTED_1 /* maybe another name */
> };
> 2. use the new enum type in simple_empty
> int simple_empty(struct dentry *dentry)
> {
> 	spin_lock(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> 	list_for_each_entry(child, &dentry->d_subdirs, d_child) {
> 		spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED_1);
> }
> 
> If you agree, I'll try to send a patch or patchset(with modification in 
> debugfs_remove_recursive).
> 

It sounds fine to me, but I think the decision needs to be with the
debugfs and vfs maintainers.

-- Steve