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

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Date:	Mon, 17 Mar 2008 23:17:41 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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
Subject: Re: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz()

(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:
> > On 03/16, Paul E. McKenney wrote:
> > > 
> > > In the process of writing up the mechanical proof of correctness for the
> > > dynticks/preemptable-RCU interface, I noticed misplaced memory barriers
> > > in rcu_enter_nohz() and rcu_exit_nohz().
> > 
> > Can't comment this patch, there is no rcu_enter_nohz() in my rcupreempt.h ;)
> 
> It is in 2.6.25-rc4 and later.  ;-)

Ah, for some reasons I'm still with -rc2 ...

> > 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.

> 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.

No?

Oleg.

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