[PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait()

[Arnd Bergmann]

From: Arnd Bergmann <arnd@xxxxxxxx>

When CONFIG_UBSAN_ALIGNMENT is enabled, the stack usage of nocb_gp_wait()
grows above typical warning limits:

In file included from kernel/rcu/tree.c:4930:
kernel/rcu/tree_nocb.h: In function 'rcu_nocb_gp_kthread':
kernel/rcu/tree_nocb.h:866:1: error: the frame size of 1968 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]

Apparently, the problem is passing rcu_data from a 'void *' pointer,
which gcc assumes may be misaligned. When the function is not inlined
into rcu_nocb_gp_kthread(), that is no longer visible to gcc.

Add a 'noinline_for_stack' annotation that leads to skipping a lot of
the alignment sanitizer checks and keeps the stack usage 60% lower here.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 kernel/rcu/tree_nocb.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 1047b30cd46b..373b877cf171 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -655,7 +655,7 @@ static void nocb_gp_sleep(struct rcu_data *my_rdp, int cpu)
  * No-CBs GP kthreads come here to wait for additional callbacks to show up
  * or for grace periods to end.
  */
-static void nocb_gp_wait(struct rcu_data *my_rdp)
+static noinline_for_stack void nocb_gp_wait(struct rcu_data *my_rdp)
 {
 	bool bypass = false;
 	int __maybe_unused cpu = my_rdp->cpu;
-- 
2.39.5