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Message-ID: <49BC8704.4020705@cn.fujitsu.com>
Date: Sun, 15 Mar 2009 12:41:40 +0800
From: Li Zefan <lizf@...fujitsu.com>
To: "Serge E. Hallyn" <serue@...ibm.com>
CC: Linux Containers <containers@...ts.linux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Paul Menage <menage@...gle.com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] devcgroup: avoid using cgroup_lock
Serge E. Hallyn wrote:
> Quoting Serge E. Hallyn (serue@...ibm.com):
>> Quoting Li Zefan (lizf@...fujitsu.com):
>>>>> @@ -426,11 +431,11 @@ static int devcgroup_access_write(struct cgroup *cgrp, struct cftype *cft,
>>>>> const char *buffer)
>>>>> {
>>>>> int retval;
>>>>> - if (!cgroup_lock_live_group(cgrp))
>>>> Does it matter that we no longer check for cgroup_is_removed()?
>>>>
>>> No, this means in a rare case that the write handler is called when the cgroup
>>> is dead, we still do the update work instead of returning ENODEV.
>>>
>>> This is ok, since at that time, accessing cgroup and devcgroup is still valid,
>>> but will have no effect since there is no task in this cgroup and the cgroup
>>> will be destroyed soon.
>> Ok, just wanted to make sure the devcgroup couldn't be partially torn
>> down and risking NULL or freed-memory derefs...
>
> Ok, so the cgroup's files will be deleted first, then on the directory
> removal the cgroup's data (each whitelist entry) is deleted. So we can
> let that ordering (by cgroup_clear_directory) ensure that nothing inside
> a file write can happen while the destroy handler is called, right?
>
When we are in the read/write handler, we have a pin in the dir's dentry
(dentry->d_count > 0), thus cgroup_diput() which destroys the cgroup won't
be called during the read/write.
> (That's why I was worried about not using the cgroup_lock: we need some
> way of synchronizing those. But I guess we're fine)
>
Many read/write handlers in other cgroup subsystems don't take cgroup_lock.
:)
>> BTW is that against linux-next? (didn't seem to apply cleanly against
>> my 2.6.29-rc9) I guess I'd like to do a little test before acking,
>> though it looks ok based on your answer.
>
> Acked-by: Serge Hallyn <serue@...ibm.com>
>
> -serge
>
>
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