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Subject: Re: [PATCH v2] rwsem: Support optimistic spinning
From: Tim Chen <tim.c.chen@linux.intel.com>
To: Davidlohr Bueso <davidlohr@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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Date: Wed, 30 Apr 2014 14:28:05 -0700
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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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