[PATCH v2] srcutree: Add WARN_ON_ONCE() for atomic SRCU in srcu_gp_end()

[Kunwu Chan <kunwu.chan@gmail.com>]

Atomic SRCU remain in the small size state, so add a
WARN_ON_ONCE() before the transition check to catch any future
code path that might violate this invariant.

Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
---
Changes since v1:
- Drop the !is_atomic guard from v1 and add WARN_ON_ONCE() instead,
  per Paul McKenney's suggestion.
- v1: https://lore.kernel.org/rcu/20260907075829.2073224-8-kunwu.chan@linux.dev/
---
 kernel/rcu/srcutree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 84c022ec8e09..68441c32cebf 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1074,6 +1074,8 @@ static void srcu_gp_end(struct srcu_struct *ssp, bool is_atomic)
 	}
 
 	/* Transition to big if needed. */
+	WARN_ON_ONCE(ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC &&
+			ss_state != SRCU_SIZE_SMALL);
 	if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {
 		if (ss_state == SRCU_SIZE_ALLOC)
 			init_srcu_struct_nodes(ssp, GFP_KERNEL);
-- 
2.43.0