Re: [PATCH tip/core/rcu 10/15] rcu: Protect rcu_node accesses during CPU stall warnings

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Aug 30, 2012 at 11:56:23AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.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@linux.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);
>  	}

Now that you've extended the lock over the rest of the loop body, I
think this would look much clearer if written without the continue and
duplicate lock release:

		...
		if (rnp->qsmask != 0)
			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
				....
		raw_spin_unlock_irqrestore(&rnp->lock, flags);
	}