Re: [PATCH 3/5] rcu/tree: Add support for debug_objects debugging for kfree_rcu()

[Paul E. McKenney]

On Tue, Aug 27, 2019 at 03:01:57PM -0400, Joel Fernandes (Google) wrote:
> Make use of RCU's debug_objects debugging support
> (CONFIG_DEBUG_OBJECTS_RCU_HEAD) similar to call_rcu() and other flavors.

Other flavors?  Ah, call_srcu(), rcu_barrier(), and srcu_barrier(),
right?

> We queue the object during the kfree_rcu() call and dequeue it during
> reclaim.
> 
> Tested that enabling CONFIG_DEBUG_OBJECTS_RCU_HEAD successfully detects
> double kfree_rcu() calls.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

The code looks good!

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9b9ae4db1c2d..64568f12641d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2757,6 +2757,7 @@ static void kfree_rcu_work(struct work_struct *work)
>  	for (; head; head = next) {
>  		next = head->next;
>  		/* Could be possible to optimize with kfree_bulk in future */
> +		debug_rcu_head_unqueue(head);
>  		__rcu_reclaim(rcu_state.name, head);
>  		cond_resched_tasks_rcu_qs();
>  	}
> @@ -2868,6 +2869,13 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  	if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING)
>  		return kfree_call_rcu_nobatch(head, func);
>  
> +	if (debug_rcu_head_queue(head)) {
> +		/* Probable double kfree_rcu() */
> +		WARN_ONCE(1, "kfree_call_rcu(): Double-freed call. rcu_head %p\n",
> +			  head);
> +		return;
> +	}
> +
>  	head->func = func;
>  
>  	local_irq_save(flags);	/* For safely calling this_cpu_ptr(). */
> -- 
> 2.23.0.187.g17f5b7556c-goog
>