Date: Thu, 5 Nov 2015 17:49:51 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH tip/locking/core v9 2/6] locking/qspinlock: prefetch next
 node cacheline
Message-ID: <20151105164951.GI3604@twins.programming.kicks-ass.net>
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On Thu, Nov 05, 2015 at 11:42:27AM -0500, Waiman Long wrote:
> If we observe next, we will observe val != tail sooner or later. It is not
> possible for it to clear the tail code in the lock. The tail xchg will
> guarantee that.
> 
> Another alternative is to do something like
> 
> +    if (!next)
>          while (!(next = READ_ONCE(node->next)))
>             cpu_relax();
> 

Yes maybe, although the main reason I fell over this was because it was
a separate change (and not mentioned in the Changelog).

Although the above would need braces (per CodingStyle), so:

	if (!next) {
		while (!(next = READ_ONCE(node->next)))
			cpu_relax();
	}


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