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

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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>
This commit is contained in:
Frederic Weisbecker 2021-01-28 18:12:08 +01:00 committed by Paul E. McKenney
parent 3820b513a2
commit 5de2e5bb80
1 changed files with 6 additions and 0 deletions
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6
kernel/rcu/tree_plugin.h
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@ -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();