linux-kernel - Re: asm-generic: Disallow no-op mb() for SMP systems

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Date:   Thu, 1 Feb 2018 15:39:51 +0000
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Stafford Horne <shorne@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        David Howells <dhowells@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: asm-generic: Disallow no-op mb() for SMP systems

On Thu, Feb 01, 2018 at 02:53:29PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 01, 2018 at 01:32:30PM +0000, Will Deacon wrote:
> > On Thu, Feb 01, 2018 at 02:29:09PM +0100, Peter Zijlstra wrote:
> > > On Thu, Feb 01, 2018 at 09:27:50PM +0900, Stafford Horne wrote:
> > > > I tried to clarify some of this in the spec v1.2 [0] which help formalize some of
> > > > the techniques we used for the SMP implementation.  Its probably not perfect,
> > > > but I added a section "10. Multicore support" and tried to clarify some things
> > > > in section 7 on Atomicity.  But it seems I dont cover exactly what are are
> > > > mentioning here.  In general:
> > > > 
> > > >   1 Secondary cores have memory snooping enabled meaning that any write to a
> > > >     cached address will cause the cache line to be invalidated.
> > > >   2 l.swa (store atomic word) implies a store buffer flush.
> > > 
> > > What about l.lwa? Can that observe 'old' values, or rather, miss values
> > > stuck in a remote store buffer?
> > > 
> > > This will then cause the first l.swa to fail, which, per the above,
> > > would then sync things up? Which means you get that one extra
> > > merry-go-round.
> > 
> > That's ok from a correctness perspective, though, as long as store buffers
> > are guaranteed to drain.
> 
> Depends a bit if you can build control dependencies off of l.swa
> succeding or not I think :-) Otherwise you get into that dodgy state you
> suffer from where bits can leak right through.
> 
> That is, I was thinking what we need for smp_mb__before_atomic.
> 
> I could've gotten my brain in a twist or course, which isn't _that_
> unusual. I never seem to be able to quite remember the holes you have
> with ll/sc on arm64 :-)

Is smp_mb__before_atomic supposed to provide ordering guarantees if it's
used before a failed cmpxchg? If so, I think it's needed here because the
l.swa might not even execute. Or did I just invent another problem?

Will