linux-kernel - Re: Re: PATCH? debugfs_remove_recursive() must not rely on list_empty(d

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Date:	Fri, 26 Jul 2013 19:24:10 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Z Lam <azl@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	David Sharp <dhsharp@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Vaibhav Nagarnaik <vnagarnaik@...gle.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Re: PATCH? debugfs_remove_recursive() must not rely on list_empty(d_subdirs)

(2013/07/26 5:04), Oleg Nesterov wrote:
> On 07/25, Oleg Nesterov wrote:
>>
>> To simplify the review, this is how it looks with the patch applied:
> 
> v2. We can use simply use list_for_each_entry_safe() and
> list_next_entry() should be calles under ->i_mutex. Although
> debugfs_remove_recursive() can race with itself anyway, but
> still.
> 
> And the code looks much simpler. But I do not know what did
> I miss.
> 
> Oleg.
> 
> void debugfs_remove_recursive(struct dentry *dentry)
> {
> 	struct dentry *child, *next, *parent;
> 
> 	if (IS_ERR_OR_NULL(dentry))
> 		return;
> 
> 	parent = dentry->d_parent;
> 	if (!parent || !parent->d_inode)
> 		return;
> 
> 	parent = dentry;
>  down:
> 	mutex_lock(&parent->d_inode->i_mutex);
> 	list_for_each_entry_safe(child, next, &parent->d_subdirs, d_u.d_child) {

Perhaps, you can use list_for_each_entry_safe_continue() here, as below.

	parent = dentry;
down:
	child = list_first_entry_or_null(&parent->d_subdirs,
					 typeof(*child), d_u.d_child);
	mutex_lock(&parent->d_inode->i_mutex);

restart:
	list_for_each_entry_safe_continue(child, next, &parent->d_subdirs, d_u.d_child) {

> 		if (!debugfs_positive(child))
> 			continue;
> 
> 		/* XXX: simple_empty(child) instead ? */
> 		if (!list_empty(&child->d_subdirs)) {
> 			mutex_unlock(&parent->d_inode->i_mutex);
> 			parent = child;
> 			goto down;
> 		}
>  up:
> 		__debugfs_remove(child, parent);
> 	}

Then, you can avoid jumping into the loop, just restart it from
parent as below.

	mutex_unlock(&parent->d_inode->i_mutex);

	child = parent;
	parent = parent->d_parent;
	mutex_lock(&parent->d_inode->i_mutex);
	__debugfs_remove(child, parent);

	if (child != dentry)
		goto restart;

> 	mutex_unlock(&parent->d_inode->i_mutex);
> 	simple_release_fs(&debugfs_mount, &debugfs_mount_count);
> }

It's just an idea, which came up to my mind. :)

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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