[PATCH 07/13] srcutree: Don't transition atomic SRCU to big in srcu_gp_end()

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

From: Kunwu Chan <kunwu.chan@gmail.com>

The transition-to-big code path in srcu_gp_end() calls
init_srcu_struct_nodes() with GFP_KERNEL, which is illegal in the
atomic context reachable from synchronize_srcu_atomic().  Atomic SRCU
has no use for the srcu_node combining tree: it has neither callbacks
nor srcu_barrier() operations, and its grace periods are serialized
by ->srcu_atomic_gp_flag instead.

Skip this transition entirely for atomic SRCU, which is also defense
in depth against any path that might wrongly set ->srcu_size_state for
an atomic srcu_struct.

Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
---
 kernel/rcu/srcutree.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 01f224a56b41..74acd5645d4c 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1074,8 +1074,9 @@ 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 (!is_atomic && 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