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

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Message-ID: <1398891992.10827.2.camel@buesod1.americas.hpqcorp.net>
Date:	Wed, 30 Apr 2014 14:06:32 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.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 14:01 -0700, Tim Chen wrote:
> On Wed, 2014-04-30 at 11:08 -0700, Tim Chen wrote:
> > On Wed, 2014-04-30 at 20:04 +0200, Peter Zijlstra wrote:
> > > On Wed, Apr 30, 2014 at 10:50:09AM -0700, Tim Chen wrote:
> > > > On Wed, 2014-04-30 at 10:27 +0200, Peter Zijlstra wrote:
> > > > > On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
> > > > > > +/*
> > > > > > + * Try to acquire write lock before the writer has been put on wait queue.
> > > > > > + */
> > > > > > +static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> > > > > > +{
> > > > > > +	long count = ACCESS_ONCE(sem->count);
> > > > > > +retry:
> > > > > > +	if (count == RWSEM_WAITING_BIAS) {
> > > > > > +		count = cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> > > > > > +			    RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS);
> > > 
> > > count = RWSEM_WAITING_BIAS
> > > new = RWSEM_WAITING_BIAS + RWSEM_ACTIVE_WRITE_BIAS
> > > new = count + RWSEM_ACTIVE_WRITE_BIAS
> > > 
> > > > > > +		/* allow write lock stealing, try acquiring the write lock. */
> > > > > > +		if (count == RWSEM_WAITING_BIAS)
> > > > > > +			goto acquired;
> > > > > > +		else if (count == 0)
> > > > > > +			goto retry;
> > > > > > +	} else if (count == 0) {
> > > > > > +		count = cmpxchg(&sem->count, 0, RWSEM_ACTIVE_WRITE_BIAS);
> > > 
> > > count = 0
> > > new = RWSEM_ACTIVE_WRITE_BIAS
> > > new = count + RWSEM_ACTIVE_WRITE_BIAS
> > > 
> > > > > > +		if (count == 0)
> > > > > > +			goto acquired;
> > > > > > +		else if (count == RWSEM_WAITING_BIAS)
> > > > > > +			goto retry;
> > > > > > +	}
> > > > > > +	return false;
> > > > > > +
> > > > > > +acquired:
> > > > > > +	return true;
> > > > > > +}
> > > > > 
> > > > > Could we have written that like:
> > > > > 
> > > > > static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> > > > > {
> > > > > 	long old, count = ACCESS_ONCE(sem->count);
> > > > > 
> > > > > 	for (;;) {
> > > > > 		if (!(count == 0 || count == RWSEM_WAITING_BIAS))
> > > > > 			return false;
> > > > > 
> > > > > 		old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS);
> > > > 
> > > > Above line won't be correct for the case when count == 0.  We are trying 
> > > > to acquire write lock, so the sem->count should become
> > > > RWSEM_ACTIVE_WRITE_BIAS, or RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS.  
> > > > So we should change the logic to
> > > > 
> > > > 		  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.

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