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

    I don't think so, I think cgroup_clone() will be called only if namespace is
    used, like clone(CLONE_NEWNS). Even if cgroup_clone() gets called, it will
    return before doing any vfs work unless the ns_cgroup subsystem is mounted.

    int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *subsys,
    char *nodename)
    {
    ...
    mutex_lock(&cgroup_mutex);
    again:
    root = subsys->root;
    if (root == &rootnode) { <--- here
    mutex_unlock(&cgroup_mutex);
    return 0;
    }

    > 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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