linux-kernel - [PATCH tip/core/rcu 03/12] rcu/nocb: Comment the reason behind BH disablement on batch processing

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Date:   Wed,  3 Mar 2021 16:23:02 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 03/12] rcu/nocb: Comment the reason behind BH disablement on batch processing

From: Frederic Weisbecker <frederic@...nel.org>

This commit explains why softirqs need to be disabled while invoking
callbacks, even when callback processing has been offloaded.  After
all, invoking callbacks concurrently is one thing, but concurrently
invoking the same callback is quite another.

Reported-by: Boqun Feng <boqun.feng@...il.com>
Reported-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree_plugin.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index cd513ea..013142d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2235,6 +2235,12 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 	local_irq_save(flags);
 	rcu_momentary_dyntick_idle();
 	local_irq_restore(flags);
+	/*
+	 * Disable BH to provide the expected environment.  Also, when
+	 * transitioning to/from NOCB mode, a self-requeuing callback might
+	 * be invoked from softirq.  A short grace period could cause both
+	 * instances of this callback would execute concurrently.
+	 */
 	local_bh_disable();
 	rcu_do_batch(rdp);
 	local_bh_enable();
-- 
2.9.5