Re: [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit

[Andy Lutomirski]

On Wed, Jun 16, 2021, at 6:37 PM, Nicholas Piggin wrote:
> Excerpts from Andy Lutomirski's message of June 17, 2021 4:41 am:
> > On 6/16/21 12:35 AM, Peter Zijlstra wrote:
> >> On Wed, Jun 16, 2021 at 02:19:49PM +1000, Nicholas Piggin wrote:
> >>> Excerpts from Andy Lutomirski's message of June 16, 2021 1:21 pm:
> >>>> membarrier() needs a barrier after any CPU changes mm.  There is currently
> >>>> a comment explaining why this barrier probably exists in all cases.  This
> >>>> is very fragile -- any change to the relevant parts of the scheduler
> >>>> might get rid of these barriers, and it's not really clear to me that
> >>>> the barrier actually exists in all necessary cases.
> >>>
> >>> The comments and barriers in the mmdrop() hunks? I don't see what is 
> >>> fragile or maybe-buggy about this. The barrier definitely exists.
> >>>
> >>> And any change can change anything, that doesn't make it fragile. My
> >>> lazy tlb refcounting change avoids the mmdrop in some cases, but it
> >>> replaces it with smp_mb for example.
> >> 
> >> I'm with Nick again, on this. You're adding extra barriers for no
> >> discernible reason, that's not generally encouraged, seeing how extra
> >> barriers is extra slow.
> >> 
> >> Both mmdrop() itself, as well as the callsite have comments saying how
> >> membarrier relies on the implied barrier, what's fragile about that?
> >> 
> > 
> > My real motivation is that mmgrab() and mmdrop() don't actually need to
> > be full barriers.  The current implementation has them being full
> > barriers, and the current implementation is quite slow.  So let's try
> > that commit message again:
> > 
> > membarrier() needs a barrier after any CPU changes mm.  There is currently
> > a comment explaining why this barrier probably exists in all cases. The
> > logic is based on ensuring that the barrier exists on every control flow
> > path through the scheduler.  It also relies on mmgrab() and mmdrop() being
> > full barriers.
> > 
> > mmgrab() and mmdrop() would be better if they were not full barriers.  As a
> > trivial optimization, mmgrab() could use a relaxed atomic and mmdrop()
> > could use a release on architectures that have these operations.
> 
> I'm not against the idea, I've looked at something similar before (not
> for mmdrop but a different primitive). Also my lazy tlb shootdown series 
> could possibly take advantage of this, I might cherry pick it and test 
> performance :)
> 
> I don't think it belongs in this series though. Should go together with
> something that takes advantage of it.

I’m going to see if I can get hazard pointers into shape quickly.

> 
> Thanks,
> Nick
>