linux-kernel - Re: [Fwd: Re: cgroup umount bug]

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Message-ID: <4E93B1CE.4070908@cn.fujitsu.com>
Date:	Tue, 11 Oct 2011 11:02:38 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	handai handai <handai.szj@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [Fwd: Re: cgroup umount bug]

>> I'm not sure from what you write whether you're aware of this, but
>> to work around this, you can then
>>
>>  mount -t cgroup -o cpu /mnt
>>  for i in `cat /mnt/test/tasks`; do
>>  echo $i > /mnt/tasks
>>  done
>>  rmdir /mnt/test
>>  umount /mnt
>>
>> and now you can
>>
>>  mount -cgroup -o cpu,cpuset cgroup /mnt
>>
> 
> I know that I can bypass this problem by deleting subgroups before
> unmount. But there is no document mentioning that I should do this to
> avoid it. But even if there is, users may still directly umount it
> accidentally, then he should try to remember what he has done and back
> to the scene to make it up, or he may need to reboot his server to do
> another test : ( .

You can check /proc/cgroups:

# mount -t cgroup -o cpu,cpuset cgroup /mnt
# mkdir /mnt/sub
# umount /mnt
# cat /proc/cgroups 
#subsys_name    hierarchy       num_cgroups     enabled
cpuset  1       2       1
cpu     1       2       1
memory  0       1       1
devices 0       1       1
freezer 0       1       1
net_cls 0       1       1
blkio   0       1       1

> So, since there is nothing good to permit incompletely unmount, why
> not prevent the potential problem by prohibiting this behavior at
> first ?
> 
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