linux-kernel - Re: [PATCH 1/2] cgroup: Delay the clearing of cgrp->kn->priv

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Date:	Thu, 4 Sep 2014 11:35:30 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Toralf Förster <toralf.foerster@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH 1/2] cgroup: Delay the clearing of cgrp->kn->priv

于 2014/9/2 23:33, Tejun Heo 写道:
> Hello, Li.
> 
> On Tue, Sep 02, 2014 at 06:56:58PM +0800, Li Zefan wrote:
>>     for ((; ;))
>>     {
>>         echo $$ > /cgroup/sub/cgroup.procs
>>         ech $$ > /cgce 6f2e0c38c2108a74 ]---
>           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>           copy & paste error?
> ...

oops

>> Reported-by: Toralf Förster <toralf.foerster@....de>
>> Signed-off-by: Li Zefan <lizefan@...wei.com>
>> ---
>>
>> Toralf, Thanks for reporting the bug. I'm not able to repy to your email,
>> because I was kicked out of the cgroup mailing list so didn't receive
>> emails from mailing list for a week.
>>
>> ---
>>  kernel/cgroup.c | 19 +++++++++----------
>>  1 file changed, 9 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>> index 1c56924..e03fc62 100644
>> --- a/kernel/cgroup.c
>> +++ b/kernel/cgroup.c
>> @@ -4185,6 +4185,15 @@ static void css_release_work_fn(struct work_struct *work)
>>  
>>  	mutex_unlock(&cgroup_mutex);
>>  
>> +	/*
>> +	 * There are two control paths which try to determine cgroup from
>> +	 * dentry without going through kernfs - cgroupstats_build() and
>> +	 * css_tryget_online_from_dir().  Those are supported by RCU
>> +	 * protecting clearing of cgrp->kn->priv backpointer.
>> +	 */
>> +	if (!ss && cgroup_parent(cgrp))
>> +		RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);
> 
> Can we move the above into the preceding else block?  I don't think
> holding cgroup_mutex or not makes any difference here. 

> Also, why do
> we need the cgroup_parent() check?  Do we deref root's kn->priv in the
> destruction path?  If so, can you please note that in the comment?
> 

I think the check is not necessary. I was trying to make smaller difference
than the original code, and RCU_INIT_POINTER() is in cgroup_rmdir() which
won't be called on root cgroup.

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