linux-kernel - Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

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Date:	Tue, 9 Jan 2007 21:29:08 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...l.org>, David Howells <dhowells@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...l.org>,
	linux-kernel@...r.kernel.org, Gautham shenoy <ego@...ibm.com>
Subject: Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

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 ...

-- 
Regards,
vatsa
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