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

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Date:	Sun, 7 Jan 2007 20:18:26 +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 05:22:46PM +0300, Oleg Nesterov wrote:
> > On 01/07, Oleg Nesterov wrote:
> > >
> > > Thoughts?
> > 
> > How about:
> > 
> > 	CPU_DEAD does nothing. After __cpu_disable() cwq->thread runs on
> > 	all CPUs and becomes idle when it flushes cwq->worklist: nobody
> 	^^^
> 
> all except dead cpus that is.

yes, of course.

> 
> > 	will add work_struct on that list.
> 
> If CPU_DEAD does nothing, then the dead cpu's workqueue list may be
> non-empty. How will it be flushed, given that no thread can run on the
> dead cpu?

But cwq->thread is not bound to the dead CPU at this point, it was aleady
migrated (like all other threads which had that CPU in ->cpus_allowed).

> 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.

I guess you misunderstood me, I meant CPU_DEAD does nothing only in
workqueue.c:workqueue_cpu_callback().

Oleg.

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