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

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Date:	Thu, 12 Feb 2009 07:07:29 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Li Zefan <lizf@...fujitsu.com>
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()

On Thu, Feb 12, 2009 at 02:54:58PM +0800, Li Zefan wrote:
> 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;
> }

Uh-oh...  That clone() will do more, actually - it will clone a bunch
of vfsmounts.  What happens if you create a separate namespace for the
first thread, so that the second one would not have our vfsmount to
play with?

Alternatively, what if the second thread is doing
	mount --bind cgroup foo
	umount foo
in a loop?

Another one: does turning the umount in the first thread into umount -l
affect anything?
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