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    SubjectRe: [RFC][PATCH 0/5] Signal scalability series
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    On Mon, 2011-10-03 at 15:16 +0200, Oleg Nesterov wrote:
    > Why do we have? Usually SIGCONT is ignored. But this doesn't matter,
    > SIGCONT acts at the sending time.
    >
    > If SIGCONT is sent - the process must not stop. Since we drop the lock
    > we can't guarantee this.

    OK, I see, thanks.

    > > > May be do_signal_stop() does something special? At first flance it doesn't.
    > > > But wait, it does while_each_thread() under ->ctrl_lock, why this is safe?
    > >
    > > Why is it not safe? What scenario are you thinking of where that isn't
    > > safe?
    >
    > This series doesn't add ->ctrl_lock into copy_process/__unhash_process
    > or I misread the patches. This means we can't trust >thread_group list.

    *facepalm*

    Arrrrggghh! This is why I complain about sighand->siglock protecting too
    much, I didn't even _REALISE_ it protected the ->thread_group list.
    Thanks for pointing that out, Oleg!

    > Even this is safe (say, we can rely on rcu), we can't calculate
    > ->group_stop_count correctly. In particular, without ->siglock we can
    > race with exit_signals() which sets PF_EXITING. Note that PF_EXITING
    > check in task_set_jobctl_pending() is important.

    Ah, I think it was these lines that confused me into thinking
    ->ctrl_lock wasn't required around PF_EXITING,

    void exit_signals(struct task_struct *tsk)
    {
    int group_stop = 0;
    sigset_t unblocked;

    if (thread_group_empty(tsk) || signal_group_exit(tsk->signal)) {
    tsk->flags |= PF_EXITING;
    return;
    }

    But I guess that's safe because either we're the only thread in the
    group or the group is already going to exit?

    --
    Matt Fleming, Intel Open Source Technology Center



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