Re: [RFC PATCH v2] membarrier: expedited private command

[Peter Zijlstra]

On Mon, Jul 31, 2017 at 11:20:59PM +1000, Michael Ellerman wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> > In fact, I'm fairly sure its only PPC.
> >
> > Because only ARM64 and PPC actually implement ACQUIRE/RELEASE with
> > anything other than smp_mb() (for now, Risc-V is in this same boat and
> > MIPS could be if they ever sort out their fancy barriers).
> >
> > TSO archs use a regular STORE for RELEASE, but all their atomics imply a
> > smp_mb() and there are enough around to make one happen (typically
> > mm_cpumask updates).
> >
> > Everybody else, aside from ARM64 and PPC must use smp_mb() for
> > ACQUIRE/RELEASE.
> >
> > ARM64 has a super duper barrier in switch_to().
> >
> > Which only leaves PPC stranded.. but the 'good' news is that mpe says
> > they'll probably need a barrier in switch_mm() in any case.
> 
> I may have been sleep deprived. We have a patch, probably soon to be
> merged, which will add a smp_mb() in switch_mm() but *only* when we add
> a CPU to mm_cpumask, ie. when we run on a CPU we haven't run on before.
> 
> I'm not across membarrier enough to know if that's sufficient, but it
> seems unlikely?

Correct, that would be insufficient. We'd need it every time switch_mm()
does indeed change the effective mm.

Now you also spoke of looking at clearing bits in mm_cpumask(), and I
suspect that if you do that, you end up having to do a barrier every
time.