linux-kernel - [PATCH 2/3] srcu: queue work without holding the lock

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Date:   Fri, 22 Sep 2017 17:28:05 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 2/3] srcu: queue work without holding the lock

On RT we can't invoke queue_delayed_work() within an atomic section
(which is provided by raw_spin_lock_irqsave()).
srcu_reschedule() invokes queue_delayed_work() outside of the
raw_spin_lock_irq_rcu_node() section so this should be fine here, too.
If the remaining callers of call_srcu() aren't atomic
(spin_lock_irqsave() is fine) then this should work on RT, too.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/rcu/srcutree.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index d190af0e56f8..3ee4ef40f23e 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -648,12 +648,17 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp,
 	/* If grace period not already done and none in progress, start it. */
 	if (!rcu_seq_done(&sp->srcu_gp_seq, s) &&
 	    rcu_seq_state(sp->srcu_gp_seq) == SRCU_STATE_IDLE) {
+		unsigned long delay;
+
 		WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
 		srcu_gp_start(sp);
+		delay = srcu_get_delay(sp);
+		raw_spin_unlock_irqrestore_rcu_node(sp, flags);
+
 		queue_delayed_work(system_power_efficient_wq, &sp->work,
-				   srcu_get_delay(sp));
-	}
-	raw_spin_unlock_irqrestore_rcu_node(sp, flags);
+				   delay);
+	} else
+		raw_spin_unlock_irqrestore_rcu_node(sp, flags);
 }
 
 /*
-- 
2.14.1