Re: [PATCH v2 0/9] Remove spin_unlock_wait()
From: Paul E. McKenney
Date: Thu Jul 06 2017 - 13:03:21 EST
On Thu, Jul 06, 2017 at 06:41:34PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 06, 2017 at 09:24:12AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 06, 2017 at 06:10:47PM +0200, Peter Zijlstra wrote:
> > > On Thu, Jul 06, 2017 at 08:21:10AM -0700, Paul E. McKenney wrote:
> > > > And yes, there are architecture-specific optimizations for an
> > > > empty spin_lock()/spin_unlock() critical section, and the current
> > > > arch_spin_unlock_wait() implementations show some of these optimizations.
> > > > But I expect that performance benefits would need to be demonstrated at
> > > > the system level.
> > >
> > > I do in fact contended there are any optimizations for the exact
> > > lock+unlock semantics.
> > You lost me on this one.
> For the exact semantics you'd have to fully participate in the fairness
> protocol. You have to in fact acquire the lock in order to have the
> other contending CPUs wait (otherwise my earlier case 3 will fail).
> At that point I'm not sure there is much actual code you can leave out.
> What actual optimization is there left at that point?
Got it. It was just that I was having a hard time parsing your sentence.
You were contending that there are no optimizations for all implementations
for the full semantics.