linux-kernel - Re: [PATCH v5 2/4] MCS Lock: optimizations and extra comments

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Message-ID: <20131119230542.GW4138@linux.vnet.ibm.com>
Date:	Tue, 19 Nov 2013 15:05:42 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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>,
	Will Deacon <will.deacon@....com>,
	"Figo.zhang" <figo1802@...il.com>
Subject: Re: [PATCH v5 2/4] MCS Lock: optimizations and extra comments

On Tue, Nov 19, 2013 at 02:57:41PM -0800, Tim Chen wrote:
> On Tue, 2013-11-19 at 11:13 -0800, Paul E. McKenney wrote:
> > On Fri, Nov 08, 2013 at 11:52:05AM -0800, Tim Chen wrote:
> > > 
> > > +/*
> > > + * Releases the lock. The caller should pass in the corresponding node that
> > > + * was used to acquire the lock.
> > > + */
> > >  static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> > >  {
> > >  	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
> > > @@ -51,7 +60,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
> > >  		/*
> > >  		 * Release the lock by setting it to NULL
> > >  		 */
> > > -		if (cmpxchg(lock, node, NULL) == node)
> > > +		if (likely(cmpxchg(lock, node, NULL) == node))
> > 
> > Agreed here as well.  Takes a narrow race to hit this.
> > 
> > So, did your testing exercise this path?  If the answer is "yes", 
> 
> 
> Paul,
> 
> I did some instrumentation and confirmed that the path in question has 
> been exercised.  So this patch should be okay.

Very good!

							Thanx, Paul

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