[PATCH tip/core/rcu 2/2] rcu: eliminate __rcu_pending() false positives

[Paul E. McKenney]

From: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Now that there are both ->gpnum and ->completed fields in the rcu_node
structure, __rcu_pending() should check rdp->gpnum and rdp->completed
against rnp->gpnum and rdp->completed, respectively, instead of the prior
comparison against the rcu_state fields rsp->gpnum and rsp->completed.
Given the old comparison, __rcu_pending() could return 1, resulting in
a needless raise_softirq(RCU_SOFTIRQ).  This useless work would happen
if RCU responded to a scheduling-clock interrupt after the rcu_state
fields had been updated, but before the rcu_node fields had been updated.

Changing the comparison from the rcu_state fields to the rcu_node fields
prevents this useless work from happening.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcutree.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1089d08..e3565cd 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1390,6 +1390,8 @@ EXPORT_SYMBOL_GPL(call_rcu_bh);
  */
 static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
 {
+	struct rcu_node *rnp = rdp->mynode;
+
 	rdp->n_rcu_pending++;
 
 	/* Check for CPU stalls, if enabled. */
@@ -1414,13 +1416,13 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
 	}
 
 	/* Has another RCU grace period completed?  */
-	if (ACCESS_ONCE(rsp->completed) != rdp->completed) { /* outside lock */
+	if (ACCESS_ONCE(rnp->completed) != rdp->completed) { /* outside lock */
 		rdp->n_rp_gp_completed++;
 		return 1;
 	}
 
 	/* Has a new RCU grace period started? */
-	if (ACCESS_ONCE(rsp->gpnum) != rdp->gpnum) { /* outside lock */
+	if (ACCESS_ONCE(rnp->gpnum) != rdp->gpnum) { /* outside lock */
 		rdp->n_rp_gp_started++;
 		return 1;
 	}
-- 
1.5.2.5

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