On Mon, Apr 02, 2007 at 01:18:28PM +0200, Ingo Molnar wrote:
> > if (freezing(current))
> > freeze_process(p); /* function exported by freezer */
>
> yeah. (is that safe with tasklist_lock held?)
from my scan of the code, it appears to be safe ..
> i'm wondering whether we could do even better than the signal approach.
> I _think_ the best approach would be to only wait for tasks that are _on
> the runqueue_. I.e. any task that has scheduled away with
> TASK_UNINTERRUPTIBLE (and might not be able to process signal events for
> a long time) is still freezable because it scheduled away.
I am slightly uncomfortable with "not waiting for tasks inside the
kernel to get out" part, even if it that is done only for
TASK_UNINTERRUPTIBLE tasks. For ex: consider this:
flush_workqueue() <- One of biggest offenders of lock_cpu_hotplug() to date
for_each_online_cpu(cpu)
flush_cpu_workqueue
TASK_UNINTERRUPTIBLE sleep
If we don't wait for this thread from being frozen "voluntarily" (because it is
in TASK_UNINTERRUPTIBLE sleep), then flush_workqueue is clearly racy wrt
cpu hotplug.
I would imagine other situations like this are possible where "not waiting
for everyone to /voluntarily/ quiece" can break cpu hotplug. In fact,
the biggest reason why we are moving to freezer based hotplug is the
fact that it quiesces everyone, leading to (hopefully) zero race conditions.
--
Regards,
vatsa
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