linux-kernel - Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636 mntput_no

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Date:	Thu, 12 Feb 2009 14:33:04 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	containers@...ts.osdl.org, Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636	mntput_no_expire+0xac/0xf2()

>>>> How cute...  Same mountpoint in both, so these mount(2) will sometimes
>>>> fail (cgroup picks the same sb on the same options, AFAICS) and fail
>>>> silently due to these redirects...
>>>>
>>>> That's a lovely way to stress-test a large part of ro-bind stuff *and*
>>>> umount()-related code.  Could you do C equivalent of the above (just
>>>> the same syscalls in loop, nothing fancier) and do time-stamped strace?
>>>>
>>> Sure, I'll write a C version and try to reproduce the warning.
>>>
>> Unfortunately, the C equivalent can't reproduce the warning, I've run the
>> test for the whole night. :( While using the script, often I can trigger
>> the warning in several mins.
> 
> Ho-hum...  I wonder if we are hitting cgroup_clone() in all that fun...

I don't think so, I think cgroup_clone() will be called only if namespace is
used, like clone(CLONE_NEWNS). Even if cgroup_clone() gets called, it will
return before doing any vfs work unless the ns_cgroup subsystem is mounted.

int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *subsys,
							char *nodename)
{
	...
	mutex_lock(&cgroup_mutex);
 again:
	root = subsys->root;
	if (root == &rootnode) {	<--- here
		mutex_unlock(&cgroup_mutex);
		return 0;
	}

> Could you
> 	a) add a printk to that sucker
> 	b) independently from (a), see if wrapping these syscalls into
> 	pid = fork();
> 	if (!pid) {
> 		[make a syscall, print something]
> 		exit(0);
> 	} else if (pid > 0) {
> 		waitpid(pid, NULL, 0);
> 	}
> and see what happens...
> 
> 
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