linux-kernel - Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks

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Message-ID: <20180706092529.GB17733@arm.com>
Date:   Fri, 6 Jul 2018 10:25:29 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Daniel Lustig <dlustig@...dia.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by
 release-acquire and by locks

On Thu, Jul 05, 2018 at 09:56:02AM -0700, Paul E. McKenney wrote:
> On Thu, Jul 05, 2018 at 05:22:26PM +0100, Will Deacon wrote:
> > On Thu, Jul 05, 2018 at 08:44:39AM -0700, Daniel Lustig wrote:
> > > On 7/5/2018 8:31 AM, Paul E. McKenney wrote:
> > > > On Thu, Jul 05, 2018 at 10:21:36AM -0400, Alan Stern wrote:
> > > >> At any rate, it looks like instead of strengthening the relation, I
> > > >> should write a patch that removes it entirely.  I also will add new,
> > > >> stronger relations for use with locking, essentially making spin_lock
> > > >> and spin_unlock be RCsc.
> > > > 
> > > > Only in the presence of smp_mb__after_unlock_lock() or
> > > > smp_mb__after_spinlock(), correct?  Or am I confused about RCsc?
> > > > 
> > > > 							Thanx, Paul
> > > > 
> > > 
> > > In terms of naming...is what you're asking for really RCsc?  To me,
> > > that would imply that even stores in the first critical section would
> > > need to be ordered before loads in the second critical section.
> > > Meaning that even x86 would need an mfence in either lock() or unlock()?
> > 
> > I think a LOCK operation always implies an atomic RmW, which will give
> > full ordering guarantees on x86. I know there have been interesting issues
> > involving I/O accesses in the past, but I think that's still out of scope
> > for the memory model.
> > 
> > Peter will know.
> 
> Agreed, x86 locked operations imply full fences, so x86 will order the
> accesses in consecutive critical sections with respect to an observer
> not holding the lock, even stores in earlier critical sections against
> loads in later critical sections.  We have been discussing tightening
> LKMM to make an unlock-lock pair order everything except earlier stores
> vs. later loads.  (Of course, if everyone holds the lock, they will see
> full ordering against both earlier and later critical sections.)
> 
> Or are you pushing for something stronger?

I (and I think Peter) would like something stronger, but we can't have
nice things ;)

Anyhow, that's not really related to this patch series, so sorry for
mis-speaking and thanks to everybody who piled on with corrections! I got
a bit arm-centric for a moment. I think Alan got the gist of it, so I'll
wait to see what he posts.

Will