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Subject[RFC PATCH 08/12] rcu: Flush bypass before setting SEGCBLIST_SOFTIRQ_ONLY
Date
Make sure to handle the pending bypass queue before we switch to the
final de-offload state. We'll have to be careful and later set
SEGCBLIST_SOFTIRQ_ONLY before re-enabling again IRQs, or new bypass
callbacks could be queued in the meantine.

Inspired-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
---
kernel/rcu/tree_plugin.h | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 3361bd351f3b..cee15841c178 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2302,11 +2302,17 @@ static void __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
SEGCBLIST_KTHREAD_GP));

+ rcu_nocb_lock_irqsave(rdp, flags);
/* Make sure nocb timer won't stay around */
- rcu_nocb_lock_irqsave(rdp, flags);
WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_OFF);
- rcu_nocb_unlock_irqrestore(rdp, flags);
del_timer_sync(&rdp->nocb_timer);
+ /*
+ * Flush bypass. While IRQs are disabled and once we set
+ * SEGCBLIST_SOFTIRQ_ONLY, no callback is supposed to be
+ * enqueued on bypass.
+ */
+ rcu_nocb_flush_bypass(rdp, NULL, jiffies);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
}

static long rcu_nocb_rdp_deoffload(void *arg)
--
2.28.0
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