Re: [PATCH] notifier: Initialize new struct srcu_usage field

[Paul E. McKenney]

On Fri, May 26, 2023 at 03:35:37PM +0800, Chen-Yu Tsai wrote:
> 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@xxxxxxxxxxxx>

Good catch, thank you!

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
> 
> 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
>