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    SubjectRe: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy
    On 04/26, Kirill Tkhai wrote:
    >
    > On 26.04.2017 18:53, Oleg Nesterov wrote:
    > >
    > >> +static long set_last_pid_vec(struct pid_namespace *pid_ns,
    > >> + struct pidns_ioc_req *req)
    > >> +{
    > >> + char *str, *p;
    > >> + int ret = 0;
    > >> + pid_t pid;
    > >> +
    > >> + read_lock(&tasklist_lock);
    > >> + if (!pid_ns->child_reaper)
    > >> + ret = -EINVAL;
    > >> + read_unlock(&tasklist_lock);
    > >> + if (ret)
    > >> + return ret;
    > >
    > > why do you need to check ->child_reaper under tasklist_lock? this looks pointless.
    > >
    > > In fact I do not understand how it is possible to hit pid_ns->child_reaper == NULL,
    > > there must be at least one task in this namespace, otherwise you can't open a file
    > > which has f_op == ns_file_operations, no?
    >
    > Sure, it's impossible to pick a pid_ns, if there is no the pid_ns's tasks. I added
    > it under impression of
    > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=dfda351c729733a401981e8738ce497eaffcaa00
    > but here it's completely wrong. It will be removed in v2.

    Hmm. But if I read this commit correctly then we really need to check
    pid_ns->child_reaper != NULL ?

    Currently we can't pick an "empty" pid_ns. But after the commit above a task
    can do sys_unshare(CLONE_NEWPID), another (or the same) task can open its
    /proc/$pid/ns/pid_for_children and call ns_ioctl() before the 1st alloc_pid() ?

    Or I am totally confused?

    Oleg.

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