linux-kernel - [PATCH rcu 14/25] rcu: Set rcu_data structures' initial ->gpwrap value to true

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Date:   Wed, 31 Aug 2022 11:11:59 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 14/25] rcu: Set rcu_data structures' initial ->gpwrap value to true

It would be good do reduce the size of the rcu_gp_oldstate structure
from three unsigned long instances to two, but this requires that the
boot-time optimized grace periods update the various ->gp_seq fields.
Updating these fields in the rcu_state structure and in all of the
rcu_node structures is at least semi-reasonable, but updating them in
all of the rcu_data structures is a bridge too far.  This means that if
there are too many early boot-time grace periods, the ->gp_seq field in
the rcu_data structure cannot be trusted.  This commit therefore sets
each rcu_data structure's ->gpwrap field to provide the necessary impetus
for a suitable level of distrust.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5c4ec9dd4ce70..03b089184b37e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -76,6 +76,7 @@
 /* Data structures. */

 static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
+	.gpwrap = true,
 #ifdef CONFIG_RCU_NOCB_CPU
 	.cblist.flags = SEGCBLIST_RCU_CORE,
 #endif
-- 
2.31.1.189.g2e36527f23