Re: [PATCH v2] rwsem: Support optimistic spinning

[Tim Chen]

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);
> > +		/* 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);
> > +		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);

> 		if (old == count)
> 			return true;
> 
> 		count = old;
> 	}
> }
> 
> ?
> 
> Just checking my brain got all the branches untangled right :-)

Thanks.

Tim

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