[cgroup or VFS ?] WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()

[Li Zefan]

>> 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?
> 

The warning still can be triggered, but seems harder (cost me 1 hour)

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

I ran following testcase, and triggered the warning in 1 hour:

thread 1:
for ((; ;))
{
        mount --bind /cgroup /mnt > /dev/null 2>&1
        umount /mnt > /dev/null 2>&1
}

tread 2:
for ((; ;))
{
        mount -t cgroup -o cpu xxx /cgroup > /dev/null 2>&1
        mkdir /cgroup/0 > /dev/null 2>&1
        rmdir /cgroup/0 > /dev/null 2>&1
        umount -l /cgroup > /dev/null 2>&1
}

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

For this one, I ran the test for the whole night, but failed to hit the warning.