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

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Message-ID: <20070109050417.GC589@in.ibm.com>
Date:	Tue, 9 Jan 2007 10:34:17 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	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 Mon, Jan 08, 2007 at 03:54:28PM -0800, Andrew Morton wrote:
> Furthermore I don't know which of these need to be tossed overboard if/when
> we get around to using the task freezer for CPU hotplug synchronisation.
> Hopefully, a lot of them.  I don't really understand why we're continuing
> to struggle with the existing approach before that question is settled.

Good point!

Fundamentally, I think we need to answer this question:

"Do we provide *some* mechanism to block concurrent hotplug operations
from happening? By hotplug operations I mean both changes to the bitmap
and execution of all baclbacks in CPU_DEAD/ONLINE etc"

If NO, then IMHO we will be forever fixing races

If YES, then what is that mechanism? freeze_processes()? or a magical
lock?

freeze_processes() cant be that mechanism, if my understanding of it is
correct - see http://lkml.org/lkml/2007/1/8/149 and 
http://marc.theaimsgroup.com/?l=linux-kernel&m=116817460726058.

I would be happy to be corrected if the above impression of
freeze_processes() is corrected ..

-- 
Regards,
vatsa
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