[PATCH rcu 10/10] rcu: Use _full() API to debug synchronize_rcu()

[Boqun Feng]

From: "Uladzislau Rezki (Sony)" <urezki@xxxxxxxxx>

Switch for using of get_state_synchronize_rcu_full() and
poll_state_synchronize_rcu_full() pair to debug a normal
synchronize_rcu() call.

Just using "not" full APIs to identify if a grace period is
passed or not might lead to a false-positive kernel splat.

It can happen, because get_state_synchronize_rcu() compresses
both normal and expedited states into one single unsigned long
value, so a poll_state_synchronize_rcu() can miss GP-completion
when synchronize_rcu()/synchronize_rcu_expedited() concurrently
run.

To address this, switch to poll_state_synchronize_rcu_full() and
get_state_synchronize_rcu_full() APIs, which use separate variables
for expedited and normal states.

Reported-by: cheung wall <zzqq0103.hey@xxxxxxxxx>
Closes: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/
Fixes: 988f569ae041 ("rcu: Reduce synchronize_rcu() latency")
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20250227131613.52683-3-urezki@xxxxxxxxx
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 include/linux/rcupdate_wait.h | 3 +++
 kernel/rcu/tree.c             | 8 +++-----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
index f9bed3d3f78d..4c92d4291cce 100644
--- a/include/linux/rcupdate_wait.h
+++ b/include/linux/rcupdate_wait.h
@@ -16,6 +16,9 @@
 struct rcu_synchronize {
 	struct rcu_head head;
 	struct completion completion;
+
+	/* This is for debugging. */
+	struct rcu_gp_oldstate oldstate;
 };
 void wakeme_after_rcu(struct rcu_head *head);
 
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 131fb463ba68..fb98d4d9d9c9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1612,12 +1612,10 @@ static void rcu_sr_normal_complete(struct llist_node *node)
 {
 	struct rcu_synchronize *rs = container_of(
 		(struct rcu_head *) node, struct rcu_synchronize, head);
-	unsigned long oldstate = (unsigned long) rs->head.func;
 
 	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
-		!poll_state_synchronize_rcu(oldstate),
-		"A full grace period is not passed yet: %lu",
-		rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
+		!poll_state_synchronize_rcu_full(&rs->oldstate),
+		"A full grace period is not passed yet!\n");
 
 	/* Finally. */
 	complete(&rs->completion);
@@ -3218,7 +3216,7 @@ static void synchronize_rcu_normal(void)
 	 * snapshot before adding a request.
 	 */
 	if (IS_ENABLED(CONFIG_PROVE_RCU))
-		rs.head.func = (void *) get_state_synchronize_rcu();
+		get_state_synchronize_rcu_full(&rs.oldstate);
 
 	rcu_sr_normal_add_req(&rs);
 
-- 
2.48.1