Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader

[Peter Zijlstra]

On Wed, Sep 08, 2021 at 01:51:24PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 01, 2021 at 11:06:27PM +0800, Boqun Feng wrote:
> @@ -201,23 +207,30 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
>  {
>  	struct rt_mutex_base *rtm = &rwb->rtmutex;
>  	unsigned long flags;
> +	int readers;
>  
>  	/* Take the rtmutex as a first step */
>  	if (rwbase_rtmutex_lock_state(rtm, state))
>  		return -EINTR;
>  
>  	/* Force readers into slow path */
> -	atomic_sub(READER_BIAS, &rwb->readers);
> +	readers = atomic_sub_return_relaxed(READER_BIAS, &rwb->readers);

Hurmph... the above really begs for something like

	if (!readers)
		return 0;

But then we needs that _acquire() thing again :/