>> 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.
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