linux-kernel - Re: [PATCH] fix misplaced mb() in rcu_enter/exit

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Message-ID: <20080317204357.GI10955@linux.vnet.ibm.com>
Date:	Mon, 17 Mar 2008 13:43:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	linux-rt-users@...r.kernel.org, mingo@...e.hu, ego@...ibm.com,
	dipankar@...ibm.com, tytso@...ibm.com, dvhltc@...ibm.com,
	akpm@...ux-foundation.org, josh@...edesktop.org,
	tglx@...utronix.de, niv@...ibm.com, heiko.carstens@...ibm.com
Subject: Re: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz()

On Mon, Mar 17, 2008 at 11:17:41PM +0300, Oleg Nesterov wrote:
> (to clarify: my question is completely offtopic to this patch)
> On 03/17, Paul E. McKenney wrote:
> > On Mon, Mar 17, 2008 at 09:30:47PM +0300, Oleg Nesterov wrote:
> > > I'm not sure the code below is up to date, but what I have in
> > > arch/s390/kernel/time.c is:
> > > 
> > > 	stop_hz_timer:
> > > 
> > > 		cpu_set(cpu, nohz_cpu_mask);
> > > 		
> > > 		if (rcu_needs_cpu(cpu) || local_softirq_pending()) {
> > > 			cpu_clear(cpu, nohz_cpu_mask);
> > > 			return;
> > > 		}
> > > 
> > > Don't we need smp_mb() after cpu_set() ?
> > 
> > S390's memory model is quite strong, so it might not be needed.
> 
> OK, in that case we shouldn't worry.

I don't know if I would go -that- far.  ;-)

> > In any
> > case, if needed, it goes -before- the cpu_set(), because the problems
> > would arise if prior RCU read-side critical sections were to be reordered
> > to follow this cpu_set(), right?
> 
> No, but it is very possible I missed something.
> 
> What if rcu_needs_cpu(cpu) is executed before cpu_set(cpu, nohz_cpu_mask)?
> It can miss rcu_start_batch() -> rcp->cur++ and return false, but at the
> same time rcu_start_batch() may see nohz_cpu_mask without this CPU.

If you mean that the rcu_needs_cpu() executes before the cpu_set() in
the code fragment above, while the rcu_start_batch() executes on some
other CPU?

Hmmm....  Can't see why this wouldn't be a problem, right off-hand,
though I cannot claim to be an s390 expert.

Heiko, thoughts?

							Thanx, Paul

> No?
> 
> Oleg.
> 
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