Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner

[Peter Zijlstra]

On Wed, Aug 24, 2016 at 05:34:12PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 24, 2016 at 10:56:59AM +0100, Will Deacon wrote:
> > > +	owner = atomic_long_read(&lock->owner);
> > > +	for (;;) {
> > > +		unsigned long old;
> > > +
> > > +		old = atomic_long_cmpxchg_release(&lock->owner, owner, owner & 0x03);
> > > +		if (old == owner)
> > > +			break;
> > > +
> > > +		owner = old;
> > > +	}
> > 
> > Can you rewrite this using atomic_long_fetch_and_release?
> 
> Yes, until patch 3/3.. but now that you mention it I think we can do:
> 
> 	owner = atomic_long_read(&lock->owner);
> 	if (!(owner & MUTEX_FLAG_HANDOFF))
> 		(void)atomic_long_fetch_and_release(MUTEX_FLAGS, &lock->owner);
> 

And of course, x86 would very much like atomic_long_and_release() here,
such that it can do LOCK ADD instead of a LOCK CMPXCHG loop. But of
course, we don't have that ...