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Date: Sat, 14 Jul 2007 23:38:17 +0400
From: Oleg Nesterov <oleg@tv-sign.ru>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Steven Rostedt <rostedt@goodmis.org>,
       Christoph Lameter <clameter@sgi.com>
Subject: Re: [PATCH -rt 5/5] slub: -rt port
Message-ID: <20070714193817.GA108@tv-sign.ru>
References: <20070714175733.194012000@chello.nl> <20070714175840.424675000@chello.nl> <20070714173950.GA806@tv-sign.ru> <1184435417.5284.58.camel@lappy>
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On 07/14, Peter Zijlstra wrote:
>
> On Sat, 2007-07-14 at 21:39 +0400, Oleg Nesterov wrote:
> > On 07/14, Peter Zijlstra wrote:
> > >
> > > +static void flush_all(struct kmem_cache *s)
> > > +{
> > > +	int cpu;
> > > +	struct workqueue_struct *wq = flush_slab_workqueue;
> > > +
> > > +	mutex_lock(&flush_slab_mutex);
> > > +	for_each_online_cpu(cpu) {
> > > +		struct slab_work_struct *sw = &per_cpu(slab_works, cpu);
> > > +
> > > +		INIT_WORK(&sw->work, flush_cpu_slab_wq);
> > > +		sw->s = s;
> > > +		queue_work_cpu(wq, &sw->work, cpu);
> > > +	}
> > > +	flush_workqueue(wq);
> > > +	mutex_unlock(&flush_slab_mutex);
> > > +}
> > 
> > I suspect this is not cpu-hotplug safe. flush_slab_mutex doesn't protect
> > from cpu_down(). This means that slab_work_struct could be scheduled on
> > the already dead CPU. flush_workqueue(wq) will hang in that case.
> 
> Yeah, the function I copied this from: schedule_on_each_cpu() has a
> comment to that effect.

Just in case, schedule_on_each_cpu() ptotects cpu_online_map with
preempt_disable(), its problem is quite different.

> Any ideas on how to solve this?

Perhaps slab_cpuup_callback() can take flush_slab_mutex too, in that
case cpu_online_map will be stable under flush_slab_mutex.

Oleg.