Re: [PATCH v4 next 3/9] locking/osq_lock: Set prev_cpu=0 instead of locked=1

[Peter Zijlstra]

On Mon, Sep 07, 2026 at 09:41:27AM +0100, David Laight wrote:

> -	for (;;) {
> -		/*
> -		 * cpu_relax() below implies a compiler barrier which would
> -		 * prevent this comparison being optimized away.
> -		 */
> +	for (;; prev = READ_ONCE(node->prev)) {
> +		if (!prev)
> +			/* Lock acquired */
> +			return true;

This lacks {}, but also, is not ACQUIRE like the return you remove
below. Did you want smp_acquire__after_ctrl_dep() in there?

> +
> +		prev_ptr = decode_cpu(prev);
> +
>  		if (data_race(prev_ptr->next) == node &&
>  		    cmpxchg(&prev_ptr->next, node, NULL) == node)
>  			break;
>  
>  		/*
> -		 * We can only fail the cmpxchg() racing against an unlock(),
> -		 * in which case we should observe @node->locked becoming
> -		 * true.
> +		 * 'prev' must have unlinked (or be in the process of unlinking)
> +		 * itself from the list.
>  		 */
> -		if (smp_load_acquire(&node->locked))
> -			return true;

This is an ACQUIRE.

>  
>  		cpu_relax();
> -
> -		/*
> -		 * Or we race against a concurrent unqueue()'s step-B, in which
> -		 * case its step-C will write us a new @node->prev pointer.
> -		 */
> -		prev = READ_ONCE(node->prev);
> -		prev_ptr = decode_cpu(prev);
>  	}