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

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Date:	Mon, 17 Mar 2008 14:09:06 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	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,
	oleg@...sign.ru, akpm@...ux-foundation.org, josh@...edesktop.org,
	tglx@...utronix.de, niv@...ibm.com
Subject: Re: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz()

On Monday 17 March 2008 12:08, Paul E. McKenney wrote:
> Hello!
>
> 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().  This patch puts them in the
> right place and adds a comment.  The key thing to keep in mind is that
> rcu_enter_nohz() is -exiting- the mode that can legally execute RCU
> read-side critical sections.  The memory barrier must be between any
> potential RCU read-side critical sections and the increment of the per-CPU
> dynticks_progress_counter, and thus must come -before- this increment.
> And vice versa for rcu_exit_nohz().
>
> The locking in the scheduler is probably saving us for the moment.
>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>
>  rcupreempt.h |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff -urpNa -X dontdiff linux-2.6.25-rc6/include/linux/rcupreempt.h
> linux-2.6.25-rc6-rcu_nohz-fix/include/linux/rcupreempt.h ---
> linux-2.6.25-rc6/include/linux/rcupreempt.h	2008-03-16 17:45:16.000000000
> -0700 +++
> linux-2.6.25-rc6-rcu_nohz-fix/include/linux/rcupreempt.h	2008-03-16
> 17:59:24.000000000 -0700 @@ -87,15 +87,15 @@ DECLARE_PER_CPU(long,
> dynticks_progress_
>
>  static inline void rcu_enter_nohz(void)
>  {
> +	mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
>  	__get_cpu_var(dynticks_progress_counter)++;
>  	WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1);
> -	mb();
>  }
>
>  static inline void rcu_exit_nohz(void)
>  {
> -	mb();
>  	__get_cpu_var(dynticks_progress_counter)++;
> +	mb(); /* CPUs seeing ++ must see subsequent RCU read-side crit sects */
>  	WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1));
>  }

Can you make these smp_mb() as well?

Thanks,
Nick

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