Subject: Re: [PATCH v2 5/7] connector/cn_proc: Protect send_msg() with a local lock

From: Ingo Molnar <[email-masked]>
Date: Mon, 25 May 2020 09:18:19 +0200

* Sebastian Andrzej Siewior <[email-masked]> wrote:

> From: Mike Galbraith <[email-masked]>
> 
> send_msg() disables preemption to avoid out-of-order messages. As the
> code inside the preempt disabled section acquires regular spinlocks,
> which are converted to 'sleeping' spinlocks on a PREEMPT_RT kernel and
> eventually calls into a memory allocator, this conflicts with the RT
> semantics.
> 
> Convert it to a local_lock which allows RT kernels to substitute them with
> a real per CPU lock. On non RT kernels this maps to preempt_disable() as
> before. No functional change.
> 
> [bigeasy: Patch description]
> 
> Cc: Evgeniy Polyakov <[email-masked]>
> Cc: [email-masked]
> Signed-off-by: Mike Galbraith <[email-masked]>
> Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
> ---
>  drivers/connector/cn_proc.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/connector/cn_proc.c b/drivers/connector/cn_proc.c
> index d58ce664da843..d424d1f469136 100644
> --- a/drivers/connector/cn_proc.c
> +++ b/drivers/connector/cn_proc.c
> @@ -18,6 +18,7 @@
>  #include <linux/pid_namespace.h>
>  
>  #include <linux/cn_proc.h>
> +#include <linux/locallock.h>
>  
>  /*
>   * Size of a cn_msg followed by a proc_event structure.  Since the
> @@ -38,25 +39,32 @@ static inline struct cn_msg *buffer_to_cn_msg(__u8 *buffer)
>  static atomic_t proc_event_num_listeners = ATOMIC_INIT(0);
>  static struct cb_id cn_proc_event_id = { CN_IDX_PROC, CN_VAL_PROC };
>  
> -/* proc_event_counts is used as the sequence number of the netlink message */
> -static DEFINE_PER_CPU(__u32, proc_event_counts) = { 0 };
> +/* local_evt.counts is used as the sequence number of the netlink message */
> +struct local_evt {
> +	__u32 counts;
> +	struct local_lock lock;
> +};
> +static DEFINE_PER_CPU(struct local_evt, local_evt) = {
> +	.counts = 0,

I don't think zero initializations need to be written out explicitly.

> +	.lock = INIT_LOCAL_LOCK(lock),
> +};
>  
>  static inline void send_msg(struct cn_msg *msg)
>  {
> -	preempt_disable();
> +	local_lock(&local_evt.lock);
>  
> -	msg->seq = __this_cpu_inc_return(proc_event_counts) - 1;
> +	msg->seq = __this_cpu_inc_return(local_evt.counts) - 1;

Naming nit: renaming this from 'proc_event_counts' to 
'local_evt.counts' is a step back IMO - what's an 'evt',
did we run out of e's? ;-)

Should be something like local_event.count? (Singular.)

Thanks,

	Ingo