Re: One potential issue with concurrent execution of RCU callbacks...

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Dec 08, 2020 at 04:54:57PM +0100, Frederic Weisbecker wrote:
> On Tue, Dec 08, 2020 at 06:58:10AM -0800, Paul E. McKenney wrote:
> > Hello, Frederic,
> > 
> > Boqun just asked if RCU callbacks ran in BH-disabled context to avoid
> > concurrent execution of the same callback.  Of course, this raises the
> > question of whether a self-posting callback can have two instances of
> > itself running concurrently while a CPU is in the process of transitioning
> > between softirq and rcuo invocation of callbacks.
> > 
> > I believe that the answer is "no" because BH-disabled context is
> > an implicit RCU read-side critical section.  Therefore, the initial
> > invocation of the RCU callback must complete in order for a new grace
> > period to complete, and a new grace period must complete before the
> > second invocation of that same callback to start.
> > 
> > Does that make sense, or am I missing something?
> 
> Sounds like a good explanation. But then why are we actually calling
> the entire rcu_do_batch() under BH-disabled context? Was it to quieten
> lockdep against rcu_callback_map ?

Inertia and lack of complaints about it.  ;-)

Plus when called from softirq, neither local_bh_disable() nor
rcu_read_lock() is necessary, and so represents pointless overhead.

> Wouldn't rcu_read_lock() around callbacks invocation be enough? Or is
> there another reason for the BH-disabled context that I'm missing?

There are likely to be callback functions that use spin_lock() instead
of spin_lock_bh() because they know that they are invoked in BH-disabled
context.

But what does this change help?

							Thanx, Paul

> Untested below:
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index bd04b09b84b3..207eff8a4e1a 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2468,6 +2468,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>  
>  		debug_rcu_head_unqueue(rhp);
>  
> +		rcu_read_lock();
>  		rcu_lock_acquire(&rcu_callback_map);
>  		trace_rcu_invoke_callback(rcu_state.name, rhp);
>  
> @@ -2476,6 +2477,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>  		f(rhp);
>  
>  		rcu_lock_release(&rcu_callback_map);
> +		rcu_read_unlock();
>  
>  		/*
>  		 * Stop only if limit reached and CPU has something to do.
> @@ -2494,11 +2496,9 @@ static void rcu_do_batch(struct rcu_data *rdp)
>  		}
>  		if (offloaded) {
>  			WARN_ON_ONCE(in_serving_softirq());
> -			local_bh_enable();
>  			lockdep_assert_irqs_enabled();
>  			cond_resched_tasks_rcu_qs();
>  			lockdep_assert_irqs_enabled();
> -			local_bh_disable();
>  		}
>  	}
>  
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index fd8a52e9a887..2a3d3c59d650 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2095,9 +2095,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>  	local_irq_save(flags);
>  	rcu_momentary_dyntick_idle();
>  	local_irq_restore(flags);
> -	local_bh_disable();
>  	rcu_do_batch(rdp);
> -	local_bh_enable();
>  	lockdep_assert_irqs_enabled();
>  	rcu_nocb_lock_irqsave(rdp, flags);
>  	if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
>