Re: numlist_pop(): Re: [RFC PATCH v4 1/9] printk-rb: add a new printk ringbuffer implementation

[Peter Zijlstra]

On Tue, Aug 20, 2019 at 10:15:18AM +0200, Petr Mladek wrote:

> 	do {
> 		tail_id = atomic_long_read(&nl->tail_id);
> 
> 		/*
> 		 * Read might fail when the tail node has been removed
> 		 * and reused in parallel.
> 		 */
> 		if (!numlist_read(nl, tail_id, NULL, &next_id))
> 			continue;
> 
> 		/* Make sure the node is not the only node on the list. */
> 		if (next_id == tail_id)
> 			return NULL;
> 
> 		/* cC: Make sure the node is not busy. */
> 		if (nl->busy(tail_id, nl->busy_arg))
> 			return NULL;
> 
> 	while (atomic_long_cmpxchg_relaxed(&nl->tail_id, tail_id, next_id) !=
> 			tail_id);

Both you and John should have a look at atomic*_try_cmpxchg*(); with
that you can write the above as:

	tail_id = atomic_long_read(&nl->tai_id);
	do {
		...
	} while (!atomic_long_try_cmpxchg_relaxed(&nl->tail_id, &tail_id, next_id));

And get better code-gen to boot.