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    SubjectRe: [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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