linux-kernel - Re: [PATCH tip/core/rcu 10/15] rcu: Protect rcu

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Message-ID: <1346943078.18408.29.camel@twins>
Date:	Thu, 06 Sep 2012 16:51:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 10/15] rcu: Protect rcu_node accesses
 during CPU stall warnings

On Thu, 2012-08-30 at 11:56 -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> The print_other_cpu_stall() function accesses a number of rcu_node
> fields without protection from the ->lock.  In theory, this is not
> a problem because the fields accessed are all integers, but in
> practice the compiler can get nasty.  Therefore, the commit extends
> the existing critical section to cover the entire loop body.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  kernel/rcutree.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 9f44749..fbe43b0 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -746,14 +746,16 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
>  	rcu_for_each_leaf_node(rsp, rnp) {
>  		raw_spin_lock_irqsave(&rnp->lock, flags);
>  		ndetected += rcu_print_task_stall(rnp);
> -		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> -		if (rnp->qsmask == 0)
> +		if (rnp->qsmask == 0) {
> +			raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  			continue;
> +		}
>  		for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
>  			if (rnp->qsmask & (1UL << cpu)) {
>  				print_cpu_stall_info(rsp, rnp->grplo + cpu);
>  				ndetected++;
>  			}
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  	}

You now cover printk() and all that that can call with a RCU lock.. is
this a good thing?
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