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    SubjectRe: [RFC PATCH(experimental) 2/2] Fix freezer-kthread_stop race
    On Sat, Apr 21, 2007 at 01:12:09AM +0400, Oleg Nesterov wrote:
    > On 04/19, Gautham R Shenoy wrote:
    > >
    > > @@ -63,12 +74,16 @@ void refrigerator(void)
    > > recalc_sigpending(); /* We sent fake signal, clean it up */
    > > spin_unlock_irq(&current->sighand->siglock);
    > >
    > > + task_lock(current);
    > > for (;;) {
    > > set_current_state(TASK_UNINTERRUPTIBLE);
    > > if (!frozen(current))
    > > break;
    > > + task_unlock(current);
    > > schedule();
    > > + task_lock(current);
    > > }
    > > + task_unlock(current);
    > > pr_debug("%s left refrigerator\n", current->comm);
    > > current->state = save;
    >
    > Just curious, why this change?
    This can race with hold_freezer_for_task() calling thaw_process. Earlier
    thaw_process(p) was called only after the process 'p' was frozen.
    Now with hold_freezer_for_task(), we can as well call thaw_process(p)
    when 'p' is in the freezing stage. Hence the task_lock.
    I know it's ugly, but couldn't think of any other alternative at that time.

    >
    > > +int hold_freezer_for_task(struct task_struct *p)
    > > +{
    > > + int ret = 0;
    > > + spin_lock(&freezer_status.lock);
    > > + if (freezer_status.count >= 0)
    > > + {
    > > + set_tsk_thread_flag(p, TIF_FREEZER_HELD);
    > > + thaw_process(p);
    > > + freezer_status.count++;
    > > + ret = 1;
    > > + }
    > > + spin_unlock(&freezer_status.lock);
    > > +
    > > + return ret;
    > > +}
    >
    > I think this can work if it is used only in kthread_stop(). But what if
    > another task wants to do hold_freezer_for_task(p) ? freezer_status.count
    > is recursive, but TIF_FREEZER_HELD is not. IOW, I believe this is not
    > generic enough.

    Yes. If more than one tasks want another task to be temporarily thawed, this
    won't work. I hadn't anticipated such a case.
    >
    > Also, you are planning to add different freezing states (FE_HOTPLUG_CPU,
    > FE_SUSPEND, etc). In that case each of them needs a separate .count, because
    > it should be negative when try_to_freeze_tasks() returns. Now consider
    > the case when we are doing freeze_processes(FE_A | FE_B) ...

    So can't we in that case find out the weight of the freeze_event variable and
    subtract that weight from the count (if the count is <=0 ) ?
    >
    > Oleg.
    >

    Thanks for the review.
    Regards
    gautham.
    --
    Gautham R Shenoy
    Linux Technology Center
    IBM India.
    "Freedom comes with a price tag of responsibility, which is still a bargain,
    because Freedom is priceless!"
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