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

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Date:	Mon, 26 Jan 2009 23:31:45 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, 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 01/26, Andrew Morton wrote:
>
> 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.

No, no helper works this way.

The reinsert doesn't make sense for cancel_work.

As for flush_work(), I think it is possible to do, but can't help to
avoid the deadlocks. Because we still have to wait for ->current_work.

> 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.

Yes.

> > 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.

Yes,

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

I meant, that

	> the caller of flush_work() can detach the work item
	> and run it directly.

this is not possible in work_on_cpu() case, we can't run it directly,
we want it to run on the target CPU.

Oleg.

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