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

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Date:	Thu, 12 Feb 2009 14:54:58 +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>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [cgroup or VFS ?] WARNING: at	fs/namespace.c:636	mntput_no_expire+0xac/0xf2()

Li Zefan wrote:
>>>>> 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.
> 

But the following testcase can also trigger the warning:

thread 1:
for ((; ;))
{
	mount -t cgroup -o ns xxx cgroup/ > /dev/null 2>&1
	# remove the dirs generated by cgroup_clone()
	rmdir cgroup/[1-9]* > /dev/null 2>&1
	umount cgroup/ > /dev/null 2>&1
}


thread 2:

int foo(void *arg)
{ return 0; }

char *stack[4096];

int main(int argc, char **argv)
{
        int usec = DEFAULT_USEC;
        while (1) {
                usleep(usec);
		# cgroup_clone() will be called
                clone(foo, stack+4096, CLONE_NEWNS, NULL);
        }

        return 0;
}

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