Re: [PATCH v2] rwsem: Support optimistic spinning

[Peter Zijlstra <peterz@infradead.org>]

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 ?