Messages in this thread | | | Date | Tue, 3 Apr 2007 22:48:20 +0530 | From | Srivatsa Vaddagiri <> | Subject | Re: [PATCH 7/8] Clean up workqueue.c with respect to the freezer based cpu-hotplug |
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On Tue, Apr 03, 2007 at 07:03:36PM +0400, Oleg Nesterov wrote: > I think it would be nice to do. I believe we can cleanup ksoftirqd() > and migration_thread() as well (kill wait_to_die: loop). Probably it
I doubt whether we can kill it in migration_thread, since that is another thread which is unfrozen for hotplug (stop_machine relies on its services while rest of the world is frozen).
> is better to introduce a new helper for that, kthread_thaw_stop() or > something.
Will think of that.
> > Why? > > What if is_single_threaded(wq) == true? In that case we should call > flush_cpu_workqueue(cpu) only if cpu == singlethread_cpu, otherwise > this is unneeded and wrong, because per_cpu_ptr(wq->cpu_wq, cpu) was > not initialized.
Ah yes ..
> > kthread_stop(p) > > { > > int old_exempt_flags; > > > > task_lock(p); > > old_exempt_flags = p->flags; > > p->flags |= PFE_ALL; /* Exempt 'p' from being frozen? */ > > I agree, we should mark this thread as non-freezable, but we can't modify > p->flags, this is racy. "current" owns its ->flags and it is not atomic. > Note that thaw_process() checks frozen(p) when it clears PF_FROZEN.
I suspected that we cannot modify p->flags just like that. How abt moving freezer exemption bits to a separate field, which is protected by task_lock?
> Actually, we should do this before destroy_workqueue() calls flush_workqueue(). > Otherwise flush_cpu_workqueue() can hang forever in a similar manner.
Yep. I guess these are a class of freezer deadlocks very similar to vfork parent waiting on child case. I get a feeling these should become common outside of kthread too (A waits on B for something, B gets frozen, which means A won't freeze causing freezer to fail). Can freezer detect this dependency somehow and thaw B automatically? Probably not that easy ..
> Needs more thinking, I guess.
[snip]
> No, no, workqueue_mutex can't help. Just for example: CPU_UP_PREPARE completes > and drops workqueue_mutex. __create_workqueue(wq) doesn't see the new cpu, it > is not on cpu_online_map, so it doesn't create cwq->thread. CPU_ONLINE oopses.
Ok ..sure.
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