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    SubjectRe: [RFC] process wide itimer cruft
    On 02/03, Peter Zijlstra wrote:
    >
    > ->live -- the number of associated tasks,
    > ->count -- not quite a refcount?

    No, ->count is not a refcount.

    Basically, ->count means how many threads didn't pass release_task() yet.
    Well, actually __exit_signal(), but this doesn't matter. The thread becomes
    "really dead" after that. Until then, it is still visible to, say,
    find_task_by_vpid, signals, etc.

    But if we have a zombie group leader, it may stay zombie "forever", and
    ->count doesn't go to zero. So we also have signal->live, when it is zero
    we know that all sub-threads at least entered do_exit(). For example,
    we can safely do exit_itimers() when ->live == 0, no other thread can
    do sys_timer_create() (or any syscall of course).

    > Could you shed a bit of light on the distinction between sighand and
    > signal?

    ->signal is protected by ->sighand->siglock, and they both cleared
    "atomically" under ->siglock in __exit_signal. I guess, the only
    reason for 2 structures is CLONE_SIGHAND which can be used without
    CLONE_THREAD.

    Now, let's look at arch/ia64/kernel/ptrace.c:ptrace_attach_sync_user_rbs()

    read_lock(&tasklist_lock);
    if (child->signal) {
    ... this task is alive, we can proceed ...

    This is correct, but if we want to make ->signal refcountable, we
    should turn the above check into

    if (child->sighand) {

    This is the same, but allows use to never clear task->signal.

    I'll try to send the patch which does this today, we should also
    change posix-cpu-timers.c and thats all, if my grepping was right.

    > > I think we really need another counter, at least for now.
    >
    > Don't rush on my account, Ingo's proposed solution doesn't need this.

    OK.

    Oleg.



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