linux-kernel - [PATCH RFC 5/8] rcu: Use rcu_node as temporary variable in funnel locking loop

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Date:   Sun, 13 May 2018 20:15:38 -0700
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <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 RFC 5/8] rcu: Use rcu_node as temporary variable in funnel locking loop

The funnel locking loop in rcu_start_this_gp uses rcu_root as a
temporary variable while walking the combining tree. This causes a
tiresome exercise of a code reader reminding themselves that rcu_root
may not be root. Lets just call it rcu_node, and then finally when
rcu_node is the rcu_root, lets assign it at that time.

Just a clean up patch, no logical change.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 kernel/rcu/tree.c | 34 ++++++++++++++++++----------------
 1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9f5679ba413b..40670047d22c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1568,7 +1568,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 {
 	bool ret = false;
 	struct rcu_state *rsp = rdp->rsp;
-	struct rcu_node *rnp_root;
+	struct rcu_node *rnp_node, *rnp_root = NULL;
 
 	/*
 	 * Use funnel locking to either acquire the root rcu_node
@@ -1581,24 +1581,26 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	 */
 	raw_lockdep_assert_held_rcu_node(rnp);
 	trace_rcu_this_gp(rnp, rdp, gp_seq_start, TPS("Startleaf"));
-	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
-		if (rnp_root != rnp)
-			raw_spin_lock_rcu_node(rnp_root);
-		if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_start) ||
-		    rcu_seq_done(&rnp_root->gp_seq, gp_seq_start) ||
-		    (rnp != rnp_root &&
-		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
-			trace_rcu_this_gp(rnp_root, rdp, gp_seq_start,
+	for (rnp_node = rnp; 1; rnp_node = rnp_node->parent) {
+		if (rnp_node != rnp)
+			raw_spin_lock_rcu_node(rnp_node);
+		if (ULONG_CMP_GE(rnp_node->gp_seq_needed, gp_seq_start) ||
+		    rcu_seq_done(&rnp_node->gp_seq, gp_seq_start) ||
+		    (rnp != rnp_node &&
+		     rcu_seq_state(rcu_seq_current(&rnp_node->gp_seq)))) {
+			trace_rcu_this_gp(rnp_node, rdp, gp_seq_start,
 					  TPS("Prestarted"));
 			goto unlock_out;
 		}
-		rnp_root->gp_seq_needed = gp_seq_start;
+		rnp_node->gp_seq_needed = gp_seq_start;
 		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
 			goto unlock_out;
-		if (rnp_root != rnp && rnp_root->parent != NULL)
-			raw_spin_unlock_rcu_node(rnp_root);
-		if (!rnp_root->parent)
+		if (rnp_node != rnp && rnp_node->parent != NULL)
+			raw_spin_unlock_rcu_node(rnp_node);
+		if (!rnp_node->parent) {
+			rnp_root = rnp_node;
 			break;  /* At root, and perhaps also leaf. */
+		}
 	}
 
 	/* If GP already in progress, just leave, otherwise start one. */
@@ -1616,10 +1618,10 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, 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_root)
-		raw_spin_unlock_rcu_node(rnp_root);
+	if (rnp != rnp_node)
+		raw_spin_unlock_rcu_node(rnp_node);
 	/* Push furthest requested GP to leaf node and rcu_data structure. */
-	if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_start)) {
+	if (ULONG_CMP_GE(rnp_node->gp_seq_needed, gp_seq_start)) {
 		rnp->gp_seq_needed = gp_seq_start;
 		rdp->gp_seq_needed = gp_seq_start;
 	}
-- 
2.17.0.441.gb46fe60e1d-goog