rcu/nocb: Comment the reason behind BH disablement on batch processing

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@gmail.com>
Reported-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index cd513ea7b0f9452fbfc534fb914b343a48c92050..013142d1ef7656355cc78717929d4fdb454664b6 100644 (file)
--- 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();