Re: [PATCH tip/core/rcu 09/15] rcu: Avoidrcu_print_detail_task_stall_rnp() segfault

[Josh Triplett]

On Thu, Aug 30, 2012 at 11:56:22AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> The rcu_print_detail_task_stall_rnp() function invokes
> rcu_preempt_blocked_readers_cgp() to verify that there are some preempted
> RCU readers blocking the current grace period outside of the protection
> of the rcu_node structure's ->lock.  This means that the last blocked
> reader might exit its RCU read-side critical section and remove itself
> from the ->blkd_tasks list before the ->lock is acquired, resulting in
> a segmentation fault when the subsequent code attempts to dereference
> the now-NULL gp_tasks pointer.
> 
> This commit therefore moves the test under the lock.  This will not
> have measurable effect on lock contention because this code is invoked
> only when printing RCU CPU stall warnings, in other words, in the common
> case, never.
> 
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcutree_plugin.h |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 139a803..c02dc1d 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -422,9 +422,11 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
>  	unsigned long flags;
>  	struct task_struct *t;
>  
> -	if (!rcu_preempt_blocked_readers_cgp(rnp))
> -		return;
>  	raw_spin_lock_irqsave(&rnp->lock, flags);
> +	if (!rcu_preempt_blocked_readers_cgp(rnp)) {
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +		return;
> +	}
>  	t = list_entry(rnp->gp_tasks,
>  		       struct task_struct, rcu_node_entry);
>  	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)

Given the small number of lines of code inside the critical section
here, I think this would look clearer without the early return and
duplicate lock release:

	raw_spin_lock_irqsave(&rnp->lock, flags);
	if (rcu_preempt_blocked_readers_cgp(rnp)) {
		...
	}
	raw_spin_unlock_irqrestore(&rnp->lock, flags);

- Josh Triplett
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