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

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Date:   Thu, 5 Jul 2018 17:22:26 +0100
From:   Will Deacon <will.deacon@....com>
To:     Daniel Lustig <dlustig@...dia.com>
Cc:     paulmck@...ux.vnet.ibm.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 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.

Will