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

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Date:   Wed, 4 Sep 2019 14:19:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: numlist_pop(): Re: [RFC PATCH v4 1/9] printk-rb: add a new
 printk ringbuffer implementation

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.