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

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Date:	Thu, 12 Feb 2009 06:24:42 +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>,
	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()

On Thu, Feb 12, 2009 at 02:10:37PM +0800, Li Zefan wrote:
> Li Zefan wrote:
> > Al Viro wrote:
> >> On Mon, Feb 09, 2009 at 12:40:46AM -0800, Andrew Morton wrote:
> >>>> Thread 1:
> >>>>   for ((; ;))
> >>>>   {
> >>>>       mount -t cgroup -o cpuset xxx /mnt > /dev/null 2>&1
> >>>>       mkdir /mnt/0 > /dev/null 2>&1
> >>>>       rmdir /mnt/0 > /dev/null 2>&1
> >>>>       umount /mnt > /dev/null 2>&1
> >>>>   }
> >>>>
> >>>> Thread 2:
> >>>>   {
> >>>>       mount -t cpuset xxx /mnt > /dev/null 2>&1
> >>>>       umount /mnt > /dev/null 2>&1
> >>>>   }
> >> 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...
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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