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    SubjectRe: [PATCH 3/3] pids: refactor vnr/nr_ns helpers to make them safe
    On Fri, Jan 16, 2009 at 09:45:40PM +0100, Oleg Nesterov wrote:
    > Hi Louis,
    >
    > On 01/16, Louis Rilling wrote:
    > >
    > > On 16/01/09 6:55 +0100, Oleg Nesterov wrote:
    > > > + struct pid_namespace *ns)
    > > > {
    > > > - return pid_nr_ns(task_pid(tsk), ns);
    > > > + pid_t nr = 0;
    > > > +
    > > > + rcu_read_lock();
    > > > + if (!ns)
    > > > + ns = current->nsproxy->pid_ns;
    > > > + if (likely(pid_alive(task))) {
    > >
    > > I don't see what this pid_alive() check buys you. Since tasklist_lock is not
    > > enforced, nothing prevents another CPU from detaching the pid right after the
    > > check.
    >
    > pid_alive() should be renamed. We use it to make sure the task didn't pass
    > __unhash_process().
    >
    > Yes, you are right, nothing prevents another CPU from detaching the pid right
    > after the check. But this is fine: we read ->pids[].pid under rcu_read_lock(),
    > and if it is NULL pid_nr_ns() returns. So, we don't need pid_alive() check at
    > all.
    >
    > However, we can not use task->group_leader unless we verify the task is still
    > alive. That is why we need this check. We do not clear ->group_leader when
    > the task exits, so we can't do
    >
    > rcu_read_lock();
    > if (task->group_leader)
    > do_something(task->group_leader);
    > rcu_unread_lock();
    >
    > Instead we use pid_alive() before using ->group_leader.

    Ok I see now. Since RCU is locked and pid_alive(task) has been true at
    some point, task->group_leader cannot be freed because release_task()
    does not release task->group_leader before releasing task. IOW
    release_task() can't have started releasing task->group_leader before
    rcu_read_lock().

    Thank you for your explanation!

    >
    > > I'm also a bit puzzled by your description with using tasklist_lock when task !=
    > > current, and not seeing tasklist_lock anywhere in the patch. Does this mean that
    > > "safe" is for "no access to freed memory is done, but caller has to take
    > > tasklist_lock or may get 0 as return value"?
    >
    > I am not sure I understand the question...
    >
    > This patch doesn't use tasklist, it relies on rcu. With this patch the caller
    > doesn't need tasklist/rcu to call these helpers (but of course, the caller
    > must ensure that task_struct is stable).
    >
    > But, whatever the caller does, it can get 0 as return value anyway if the
    > task exists, this is correct. Or I misunderstood you?

    My question was probably badly phrased. When reading the patch
    description I thought that the point was to fix the helpers so that
    tasklist_lock would be taken whenever task != current. Of course your
    patch does not do this, and I'm perfectly fine with this.

    Thanks,

    Louis

    --
    Dr Louis Rilling Kerlabs - IRISA
    Skype: louis.rilling Campus Universitaire de Beaulieu
    Phone: (+33|0) 2 99 84 71 52 Avenue du General Leclerc
    Fax: (+33|0) 2 99 84 71 71 35042 Rennes CEDEX - France
    http://www.kerlabs.com/


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