netdev - Re: [PATCH RFC] v5 expedited "big hammer" RCU grace periods

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Date:	Mon, 18 May 2009 07:40:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, mingo@...e.hu,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	davem@...emloft.net, dada1@...mosbay.com, zbr@...emap.net,
	jeff.chua.linux@...il.com, paulus@...ba.org, jengelh@...ozas.de,
	r000n@...0n.net, benh@...nel.crashing.org,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: [PATCH RFC] v5 expedited "big hammer" RCU grace periods

On Mon, May 18, 2009 at 02:59:52PM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:
> > +void sched_expedited_wake(void *unused)
> > +{
> > +	mutex_lock(&__get_cpu_var(sched_expedited_done_mutex));
> > +	if (__get_cpu_var(sched_expedited_done_qs) ==
> > +	    SCHED_EXPEDITED_QS_DONE_QS) {
> > +		__get_cpu_var(sched_expedited_done_qs) =
> > +			SCHED_EXPEDITED_QS_NEED_QS;
> > +		wake_up(&__get_cpu_var(sched_expedited_qs_wq));
> > +	}
> > +	mutex_unlock(&__get_cpu_var(sched_expedited_done_mutex));
> > +}
> 
> [...]
> 
> > +		get_online_cpus();
> > +		preempt_disable();
> > +		mycpu = smp_processor_id();
> > +		smp_call_function(sched_expedited_wake, NULL, 1);
> 
> sched_expedited_wake() calls mutex_lock() which may sleep?

Good eyes!  Fixing this and the failure to release this lock in
krcu_sched_expedited_percpu() allows it to survive 10 hours of
rcutorture running in parallel with onlining/offlining random CPUs.

> And I think you have re-implement workqueue.

Hmmm...  I suppose I could use schedule_work(), though I am concerned
about interference from other work.  But I will give this some thought.

							Thanx, Paul
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