linux-kernel - Re: [PATCH, RFC] reimplement flush

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Message-ID: <20070104143856.GB179@tv-sign.ru>
Date:	Thu, 4 Jan 2007 17:38:56 +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, RFC] reimplement flush_workqueue()

On 01/04, Srivatsa Vaddagiri wrote:
>
> On Mon, Dec 18, 2006 at 01:34:16AM +0300, Oleg Nesterov wrote:
> >  void fastcall flush_workqueue(struct workqueue_struct *wq)
> >  {
> > -	might_sleep();
> > -
> > +	mutex_lock(&workqueue_mutex);
> >  	if (is_single_threaded(wq)) {
> >  		/* Always use first cpu's area. */
> > -		flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu),
> > -					-1);
> > +		flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu));
> >  	} else {
> >  		int cpu;
> > 
> > -		mutex_lock(&workqueue_mutex);
> >  		for_each_online_cpu(cpu)
> 
> 
> Can compiler optimizations lead to cpu_online_map being cached in a register 
> while running this loop? AFAICS cpu_online_map is not declared to be
> volatile.

But it is not const either,

>            If it can be cached,

I believe this would be a compiler's bug. Let's take a more simple example,

	while (!condition)
		schedule();

What if compiler will cache the value of global 'condition' ?

                                  then we have the danger of invoking 
> flush_cpu_workqueue() on a dead cpu (because flush_cpu_workqueue drops
> workqueue_mutex, cpu hp events can change cpu_online_map while we are in
> flush_cpu_workqueue).

Oleg.

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