linux-kernel - Re: [PATCH] fix-flush_workqueue-vs-cpu

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Date:	Sun, 7 Jan 2007 20:33:34 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Srivatsa Vaddagiri <vatsa@...ibm.com>
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] fix-flush_workqueue-vs-cpu_dead-race-update

On 01/07, Srivatsa Vaddagiri wrote:
>
> On Sun, Jan 07, 2007 at 10:13:44PM +0530, Srivatsa Vaddagiri wrote:
> 
> I guess you could have cwq->thread flush only it's cpu's workqueue by
> running on another cpu,

yes, this is what I meant,

>                           which will avoid the need to synchronize
> between worker threads. I am not 100% sure if that breaks workqueue
> model in any way (since we could have two worker threads running on the
> same CPU, but servicing different queues). Hopefully it doesnt.

We are already doing this on CPU_DEAD->kthread_stop().

> However the concern expressed below remains ..
> 
> > Finally, I am concerned about the (un)friendliness of this programming
> > model, where programmers are restricted in not having a stable access to
> > cpu_online_map at all -and- also requiring them to code in non-obvious
> > terms. Granted that writing hotplug-safe code is non-trivial, but the
> > absence of "safe access to online_map" will make it more complicated.

please see the previous message.

Srivatsa, I don't claim my idea is the best. Actually I still hope somebody
else will suggest something better and simpler :)

Oleg.

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