linux-kernel - [PATCH tip/core/rcu 2/9] rcu: Correctly unlock root node in rcu_check_gp_start

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Date:   Thu, 30 May 2019 08:00:08 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 2/9] rcu: Correctly unlock root node in rcu_check_gp_start_stall()

From: Neeraj Upadhyay <neeraju@...eaurora.org>

On systems whose rcu_node tree has only one node, the
rcu_check_gp_start_stall() function's values of rnp and rnp_root will
be identical.  In this case, it clearly does not make sense to release
both rnp->lock and rnp_root->lock, but that is exactly what this function
does in the last early exit.  This commit therefore unlocks only rnp->lock
when rnp and rnp_root are equal.

Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
Reviewed-by: Mukesh Ojha <mojha@...eaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/rcu/tree_stall.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index f65a73a97323..065183391f75 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -630,7 +630,9 @@ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
 	    time_before(j, rcu_state.gp_req_activity + gpssdelay) ||
 	    time_before(j, rcu_state.gp_activity + gpssdelay) ||
 	    atomic_xchg(&warned, 1)) {
-		raw_spin_unlock_rcu_node(rnp_root); /* irqs remain disabled. */
+		if (rnp_root != rnp)
+			/* irqs remain disabled. */
+			raw_spin_unlock_rcu_node(rnp_root);
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 		return;
 	}
-- 
2.17.1