Message-ID: <1391452760.7498.26.camel@j-VirtualBox>
Subject: Re: [RFC][PATCH v2 5/5] mutex: Give spinners a chance to
 spin_on_owner if need_resched() triggered while queued
From: Jason Low <jason.low2@hp.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>, Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Waiman Long <Waiman.Long@hp.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Rik van Riel <riel@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Davidlohr Bueso <davidlohr@hp.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Andi Kleen <andi@firstfloor.org>,
        "Chandramouleeswaran, Aswin" <aswin@hp.com>,
        "Norton, Scott J" <scott.norton@hp.com>, chegu_vinod@hp.com
Date: Mon, 03 Feb 2014 10:39:20 -0800
In-Reply-To: <20140202211230.GX5002@laptop.programming.kicks-ass.net>
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On Sun, 2014-02-02 at 22:12 +0100, Peter Zijlstra wrote:
> The way I wrote that same loop in step-B, is:
> 
> 
> 	for (;;) {
> 		if (*lock == node && cmpxchg(lock, node, prev) == node)
> 			return
> 
> 		next = xchg(&node->next, NULL); /* B -> A */
> 		if (next)
> 			break;
> 
> 		arch_mutex_cpu_relax();
> 	}
> 
> I suppose we can make that something like:
> 
> 
> 	if (node->next) {
> 		next = xchg(&node->next, NULL);
> 		if (next)
> 			break
> 	}
> 
> To avoid the xchg on every loop.

Ah yes, we want to use xchg() on &node->next.

Since the cmpxchg() is now in a loop in the unlock function, an
additional (*lock == node) check before the cmpxchg() would also be nice
to avoid spinning on cmpxchg() there too.

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