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

[Joel Fernandes]

On Wed, Aug 28, 2019 at 02:31:19PM -0700, Paul E. McKenney wrote:
> 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?

Yes.

> > 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!

thanks, does that mean you'll ack/apply it? :-P

 - Joel

> 
> 							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
> >