[PATCH v4 for 4.14 1/3] membarrier: Provide register expedited private command

[Nicholas Piggin]

On Thu, 28 Sep 2017 15:29:50 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote:

> ----- On Sep 28, 2017, at 11:01 AM, Nicholas Piggin npiggin at gmail.com wrote:
> 
> > On Thu, 28 Sep 2017 13:31:36 +0000 (UTC)
> > Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote:
> >   
> >> ----- On Sep 27, 2017, at 9:04 AM, Nicholas Piggin npiggin at gmail.com wrote:
> >>   

[snip]

> >> So I don't see much point in trying to remove that registration step.  
> > 
> > I don't follow you. You are talking about the concept of registering
> > intention to use a different function? And the registration API is not
> > merged yet?  
> 
> Yes, I'm talking about requiring processes to invoke membarrier cmd
> MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED before they can successfully
> invoke membarrier cmd MEMBARRIER_CMD_PRIVATE_EXPEDITED.
> 
> > Let me say I'm not completely against the idea of a registration API. But
> > don't think registration for this expedited command is necessary.  
> 
> Given that we have the powerpc lack-of-full-barrier-on-return-to-userspace
> case now, and we foresee x86-sysexit, sparc, and alpha also requiring
> special treatment when we introduce the MEMBARRIER_FLAG_SYNC_CORE behavior
> in the next release, it seems that we'll have a hard time handling
> architecture special cases efficiently if we don't expose the registration
> API right away.

But SYNC_CORE is a different functionality, right? You can add the
registration API for it when that goes in.

> > But (aside) let's say a tif flag turns out to be a good diea for your
> > second case, why not just check the flag in the membarrier sys call and
> > do the registration the first time it uses it?  
> 
> We also considered that option. It's mainly about guaranteeing that
> an expedited membarrier command never blocks. If we introduce this
> "lazy auto-registration" behavior, we end up blocking the process
> at a random point in its execution so we can issue a synchronize_sched().
> By exposing an explicit registration, we can control where this delay
> occurs, and even allow library constructors to invoke the registration
> while the process is a single threaded, therefore allowing us to completely
> skip synchronize_sched().

Okay I guess that could be a good reason. As I said I'm not opposed to
the concept. I suppose you could even have a registration for expedited
private even if it's a no-op on all architectures, just in case some new
ways of implementing it can be done in future.

I suppose I'm more objecting to the added complexity for powerpc, and
more code in the fastpath to make the slowpath faster.

Thanks,
Nick