Messages in this thread | | | Date | Tue, 9 Jan 2007 21:29:08 +0530 | From | Srivatsa Vaddagiri <> | Subject | Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist |
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On Tue, Jan 09, 2007 at 06:07:55PM +0300, Oleg Nesterov wrote: > but at some point we should thaw processes, including cwq->thread which > should die.
I am presuming we will thaw processes after all CPU_DEAD handlers have run.
> So we are doing things like take_over_work() and this is the > source of races, because the dead CPU is not on cpu_online_map. > > flush_workqueue() doesn't use any locks now. If we use freezer to implement > cpu-hotplug nothing will change, we still have races.
We have races -if- CPU_DEAD handling can run concurrently with a ongoing flush_workqueue. From my recent understanding of process freezer, this is not possible. In other words, flush_workqueue() can be its old implementation as below w/o any races:
some_thread:
for_each_online_cpu(i) flush_cpu_workqueue(i);
As long as this loop is running, cpu_down/up will not proceed. This means, cpu_online_map is stable even if flush_cpu_workqueue blocks ..
Once this loop is complete and all threads have called try_to_freeze, cpu_down will proceed to change the bit map and run CPU_DEAD handlers of everyone. I am presuimg we will thaw processes only after all CPU_DEAD/ONLINE handlers have run (dont know if that is a problem). In that case do you still see races? Yes, this would require some changes in worker_thread to check for kthread_should_stop() after try_to_freeze returns ...
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