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) &&
