[PATCH] notifier: Initialize new struct srcu_usage field

[Chen-Yu Tsai]

In commit 95433f726301 ("srcu: Begin offloading srcu_struct fields to
srcu_update"), a new struct srcu_usage field was added, but was not
properly initialized. This led to a "spinlock bad magic" BUG when
the SRCU notifier was ever used. This was observed in the MediaTek
CCI devfreq driver on next-20230525. Trimmed stack trace as follows:

    BUG: spinlock bad magic on CPU#4, swapper/0/1
     lock: 0xffffff80ff529ac0, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
    Call trace:
     spin_bug+0xa4/0xe8
     do_raw_spin_lock+0xec/0x120
     _raw_spin_lock_irqsave+0x78/0xb8
     synchronize_srcu+0x3c/0x168
     srcu_notifier_chain_unregister+0x5c/0xa0
     cpufreq_unregister_notifier+0x94/0xe0
     devfreq_passive_event_handler+0x7c/0x3e0
     devfreq_remove_device+0x48/0xe8

Add __SRCU_USAGE_INIT() to SRCU_NOTIFIER_INIT() so that srcu_usage gets
initialized properly.

Fixes: 95433f726301 ("srcu: Begin offloading srcu_struct fields to srcu_update")
Signed-off-by: Chen-Yu Tsai <wenst at chromium.org>
---

Also, the original patch subject said "srcu_update", however the data
structure ended up as "srcu_usage". Maybe that could be updated?

 include/linux/notifier.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 2aba75145144..86544707236a 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -106,12 +106,22 @@ extern void srcu_init_notifier_head(struct srcu_notifier_head *nh);
 #define RAW_NOTIFIER_INIT(name)	{				\
 		.head = NULL }
 
+#ifdef CONFIG_TREE_SRCU
 #define SRCU_NOTIFIER_INIT(name, pcpu)				\
 	{							\
 		.mutex = __MUTEX_INITIALIZER(name.mutex),	\
 		.head = NULL,					\
+		.srcuu = __SRCU_USAGE_INIT(name.srcuu),		\
 		.srcu = __SRCU_STRUCT_INIT(name.srcu, name.srcuu, pcpu), \
 	}
+#else
+#define SRCU_NOTIFIER_INIT(name, pcpu)				\
+	{							\
+		.mutex = __MUTEX_INITIALIZER(name.mutex),	\
+		.head = NULL,					\
+		.srcu = __SRCU_STRUCT_INIT(name.srcu, name.srcuu, pcpu), \
+	}
+#endif
 
 #define ATOMIC_NOTIFIER_HEAD(name)				\
 	struct atomic_notifier_head name =			\
-- 
2.41.0.rc0.172.g3f132b7071-goog