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

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Date:   Fri, 15 Nov 2019 09:57:12 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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



在 2019/11/15 9:47, yukuai (C) 写道:
> 
> 
> 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
missing a ','
>      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);
should be 'spin_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).
sorry about the stupid mistake..

Thanks!
Yu Kuai