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    SubjectRe: [RFC PATCH(experimental) 2/2] Fix freezer-kthread_stop race
    On Mon, Apr 23, 2007 at 10:39:56PM +0400, Oleg Nesterov wrote:
    > On 04/23, Gautham R Shenoy wrote:
    > >
    > > 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.
    >
    > hold_freezer_for_task()->thaw_process(p) will wake up the task. Or the
    > caller of refrigerator will notice "!frozen()". Note that refrigerator()
    > sets PF_FROZEN under task_lock().
    >
    > In fact we have the same issue when thaw_tasks()->thaw_process(p) happens
    > when the freezing fails. In that case 'p' may be not frozen.

    Yes. I guess I was being too cautious :)

    >
    > > > 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 ) ?
    >
    > Probably yes... but if we are speaking about kthrad_stop() only, this could
    > be afaics solved in more simple way, as Rafael suggests.

    I agree on this one. kthread_stop appears to be the only valid place
    where such a correction is required. So Rafael's approach makes sense
    here.

    >
    > Oleg.
    >
    Thanks and 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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