[PATCH v3] srcutree: Don't transition atomic SRCU to big in srcu_gp_end()

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

Atomic SRCU must remain in the small size state. Warn if this
invariant is violated and avoid transitioning to big in that case.

Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
---
Changes since v2:
- Also prevent the transition when the atomic SRCU size-state
  invariant is violated, per Paul McKenney's suggestion.
-v2: https://lore.kernel.org/rcu/20260910084629.3695383-1-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 | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 84c022ec8e09..07db4e459b30 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1073,8 +1073,11 @@ static void srcu_gp_end(struct srcu_struct *ssp, bool is_atomic)
 		raw_spin_unlock_irq_rcu_node(sup);
 	}
 
-	/* Transition to big if needed. */
-	if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {
+	/* Transition to big if needed, but never for atomic SRCU. */
+	if (ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC &&
+	    ss_state != SRCU_SIZE_SMALL) {
+		WARN_ON_ONCE(1);
+	} else if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {
 		if (ss_state == SRCU_SIZE_ALLOC)
 			init_srcu_struct_nodes(ssp, GFP_KERNEL);
 		else
-- 
2.43.0