Messages in this thread | | | Date | Mon, 8 Jan 2007 20:06:35 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update |
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On 01/08, Srivatsa Vaddagiri wrote: > > On Mon, Jan 08, 2007 at 06:56:38PM +0300, Oleg Nesterov wrote: > > > 2. > > > > > > CPU_DEAD->cleanup_workqueue_thread->(cwq->thread = NULL)->kthread_stop() .. > > > ^^^^^^^^^^^^^^^^^^^^ > > > |___ Problematic > > > > Hmm... This should not be possible? cwq->thread != NULL on CPU_DEAD event. > > sure, cwq->thread != NULL at CPU_DEAD event. However > cleanup_workqueue_thread() will set it to NULL and block in > kthread_stop(), waiting for the kthread to finish run_workqueue and > exit.
Ah, missed you point, thanks. Yet another old problem which was not introduced by recent changes. And yet another indication we should avoid kthread_stop() on CPU_DEAD event :) I believe this is easy to fix, but need to think more.
> > > A lock_cpu_hotplug(), or any other ability to block concurrent hotplug > > > operations from happening, in run_workqueue would have avoided both the above > > > races. > > > > I still don't think this is a good idea. We also need > > is_cpu_down_waits_for_lock_cpu_hotplug() > > > > helper, otherwise we have a deadlock if work->func() sleeps and re-queues itself. > > Can you elaborate this a bit?
If work->func() re-queues itself, run_workqueue() never returns because ->worklist is never empty. This means we should somehow check and detect that cpu-hotplug blocked because we hold lock_cpu_hotplug(). In that case run_workqueue() should return, and drop the lock. This will complicate worker_thread/run_workqueue further.
run_workqueue:
while (!list_empty(&cwq->worklist)) { ... // We hold lock_cpu_hotplug(), cpu event can't make // progress. ... }
> > Yes, http://marc.theaimsgroup.com/?l=linux-kernel&m=116818097927685, I believe > > we can do this later. This way workqueue will have almost zero interaction > > with cpu-hotplug, and cpu UP/DOWN event won't be delayed by sleeping work.func(). > > take_over_work() can go away, this also allows us to simplify things. > > I agree it minimizes the interactions. Maybe worth attempting. However I > suspect it may not be as simple as it appears :)
Yes, that is why this patch only does the first step: flush_workqueue() checks the dead CPUs as well, this change is minimal.
Do you see any problems this patch adds?
Oleg.
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