linux-kernel - Re: [PATCH v2] rwsem: Support optimistic spinning

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Message-ID: <1398893285.2970.111.camel@schen9-DESK>
Date:	Wed, 30 Apr 2014 14:28:05 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rwsem: Support optimistic spinning

On Wed, 2014-04-30 at 14:06 -0700, Davidlohr Bueso wrote:

> > > > > 
> > > > > 		  if (count == RWSEM_WAITING_BIAS)
> > > > > 			old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS);
> > > > > 		  else
> > > > > 			old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
> > > > 
> > > > I think I simply mis-typed it; shouldn't both cases be
> > > > RWSEM_ACTIVE_WRITE_BIAS ?
> > > 
> > > Yeah,  we should just write it as
> > > 			  old = cmpxchg(&sem->count, count, RWSEM_ACTIVE_WRITE_BIAS);
> > 
> > Oops, I mean
> > 			    old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
> > 
> > for count == 0, we need sem->count to be RWSEM_ACTIVE_WRITE_BIAS, 
> > for count == RWSEM_WAITING_BIAS, we need sem->count to be  RWSEM_WAITING_BIAS + RWSEM_ACTIVE_WRITE_BIAS
> 
> Yep, I had just noticed this. Peter's original suggestion was correct,
> just needed to change RWSEM_ACTIVE_BIAS for RWSEM_ACTIVE_WRITE_BIAS.
> 

Yes, I think we are all on the same page now.

Tim

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