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

[Frederic Weisbecker <frederic@kernel.org>]

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 ?

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

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) &&