linux-kernel - Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

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Message-Id: <20090126141605.707877bb.akpm@linux-foundation.org>
Date:	Mon, 26 Jan 2009 14:16:05 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	oleg@...hat.com, a.p.zijlstra@...llo.nl, rusty@...tcorp.com.au,
	travis@....com, mingo@...hat.com, davej@...hat.com,
	cpufreq@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

On Mon, 26 Jan 2009 23:05:37 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > Well it turns out that I was having a less-than-usually-senile moment:
> > 
> > :     implement flush_work()
> 
> > Why isn't that working in this case??
> 
> how would that work in this case? We defer processing into the workqueue 
> exactly because we want its per-CPU properties.

It detaches the work item, moves it to head-of-queue, reinserts it then
waits on it.  I think.

This might have a race+hole.  If a currently-running "unrelated"
work item tries to take the lock which the flush_work() caller is holding
then there's no way in which keventd will come back to execute
the work item which we just put on the head of queue.

> We want work_on_cpu() to 
> be done in the workqueue context on the CPUs that were specified, not in 
> the local CPU context.

flush_work() is supposed to work in the way which you describe.

But Oleg's "we may be running on a different CPU" comment has me all
confused.

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