linux-kernel - [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq

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Date:   Sun, 20 May 2018 21:42:20 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, byungchul.park@....com,
        kernel-team@...roid.com
Subject: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq_needed

We acquire gp_seq_needed locklessly. To be safe, lets do the unlocking
after the access.

Signed-off-by: Joel Fernandes <joel@...lfernandes.org>
---
 kernel/rcu/tree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 879c67a31116..efbd21b2a1a6 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1603,13 +1603,13 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
 	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
 	ret = true;  /* Caller must wake GP kthread. */
 unlock_out:
-	if (rnp != rnp_start)
-		raw_spin_unlock_rcu_node(rnp);
 	/* Push furthest requested GP to leaf node and rcu_data structure. */
 	if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req)) {
 		rnp_start->gp_seq_needed = gp_seq_req;
 		rdp->gp_seq_needed = gp_seq_req;
 	}
+	if (rnp != rnp_start)
+		raw_spin_unlock_rcu_node(rnp);
 	return ret;
 }
 
-- 
2.17.0.441.gb46fe60e1d-goog