linux-kernel - Re: [PATCH v7 2/6] MCS Lock: optimizations and extra comments

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Date:	Mon, 20 Jan 2014 11:11:25 -0800
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Waiman Long <waiman.long@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	"Figo.zhang" <figo1802@...il.com>
Subject: Re: [PATCH v7 2/6] MCS Lock: optimizations and extra comments

On Mon, 2014-01-20 at 14:58 +0100, Peter Zijlstra wrote:
> On Thu, Jan 16, 2014 at 04:08:20PM -0800, Tim Chen wrote:
> > Remove unnecessary operation and make the cmpxchg(lock, node, NULL) == node
> > check in mcs_spin_unlock() likely() as it is likely that a race did not occur
> > most of the time.
> 
> It might be good to describe why the node->locked=1 is thought
> unnecessary. I concur it is, but upon reading this changelog I was left
> wondering and had to go read the code and run through the logic to
> convince myself.
> 
> Having done so, I'm now wondering if we think so for the same reason --
> although I'm fairly sure we are.
> 
> The argument goes like: everybody only looks at his own ->locked value,
> therefore the only one possibly interested in node->locked is the lock
> owner. However the lock owner doesn't care what's in it, it simply
> assumes its 1 but really doesn't care one way or another.

Yes, it is done for the same reason you mentioned.  I'll update
the comments better to reflect this.

> 
> That said, a possible DEBUG mode might want to actually set it, validate
> that all other linked nodes are 0 and upon unlock verify the same before
> flipping next->locked to 1.

I'll leave a comment to indicate this. If we need a DEBUG mode later,
we can come back to add this easily.

Thanks.

Tim



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