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

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Sep 08, 2021 at 01:51:24PM +0200, Peter Zijlstra 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);
>  
> -	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
>  	/*
>  	 * set_current_state() for rw_semaphore
>  	 * current_save_and_set_rtlock_wait_state() for rwlock
>  	 */
>  	rwbase_set_and_save_current_state(state);
> +	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
>  
> -	/* Block until all readers have left the critical section. */
> -	for (; atomic_read(&rwb->readers);) {
> +	/*
> +	 * Block until all readers have left the critical section.
> +	 *
> +	 * In the case of !readers, the above implies TSO ordering
> +	 * at the very least and hence provides ACQUIRE vs the earlier
> +	 * atomic_sub_return_relaxed().
> +	 */
> +	while (readers) {
>  		/* Optimized out for rwlocks */
>  		if (rwbase_signal_pending_state(state, current)) {
>  			__set_current_state(TASK_RUNNING);
> @@ -230,8 +243,12 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
>  		 * Schedule and wait for the readers to leave the critical
>  		 * section. The last reader leaving it wakes the waiter.
>  		 */
> -		if (atomic_read(&rwb->readers) != 0)
> +		readers = atomic_read(&rwb->readers);
> +		if (readers != 0)
>  			rwbase_schedule();
> +		/*
> +		 * Implies smp_mb() and provides ACQUIRE for the !readers case.
> +		 */

->readers may get changed to non-zero here, because ->wait_lock is not
held by the writer, and there could be readers in slow-path running.
We need to re-read ->readers after holding ->wait_lock. Otherwise, we
may use an old value of ->readers, and grab a write lock while there
still exists readers.

Regards,
Boqun

>  		set_current_state(state);
>  		raw_spin_lock_irqsave(&rtm->wait_lock, flags);
>  	}
[...]