linux-kernel - Re: [PATCH, RFC] reimplement flush

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Message-ID: <20070106151153.GA24274@in.ibm.com>
Date:	Sat, 6 Jan 2007 20:41:53 +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, RFC] reimplement flush_workqueue()

On Fri, Jan 05, 2007 at 03:42:46PM +0300, Oleg Nesterov wrote:
> preempt_disable() can't prevent cpu_up, but flush_workqueue() doesn't care
> _unless_ cpu_down also happened meantime (and hence a fresh CPU may have
> pending work_structs which were moved from a dead CPU).

Yes, that was what I had in mind.

> So you are right, we still need the patch above, but I think we don't have
> new problems with preempt_disable().

Right, preempt_disable() hasn't added any new problem. Its just
revealing the same problem as earlier, by opening up window for cpu
hotplug events to happen in the middle of flush_workqueue().

Ideally I would have liked a lock_cpu_hotplug() equivalent which blocks
all cpu hotplug events during the entire flush_workqueue(). In its
absence, I guess we just need to deal with all these ugly races ..

In summary, I think we need to go ahead with the preemp_disable() patch
in flush_workqueue() from Andrew and the race fix you posted here:

	http://lkml.org/lkml/2006/12/30/37

-- 
Regards,
vatsa
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