linux-kernel - Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

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Date:   Wed, 11 Jul 2018 01:14:55 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.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>,
        Will Deacon <will.deacon@....com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire

On Tue, Jul 10, 2018 at 11:34:45AM -0400, Alan Stern wrote:
> On Tue, 10 Jul 2018, Andrea Parri wrote:
> 
> > > >   ACQUIRE operations include LOCK operations and both smp_load_acquire()
> > > >   and smp_cond_acquire() operations.  [BTW, the latter was replaced by
> > > >   smp_cond_load_acquire() in 1f03e8d2919270 ...]
> > > > 
> > > >   RELEASE operations include UNLOCK operations and smp_store_release()
> > > >   operations. [...]
> > > > 
> > > >   [...] after an ACQUIRE on a given variable, all memory accesses
> > > >   preceding any prior RELEASE on that same variable are guaranteed
> > > >   to be visible.
> > > 
> > > As far as I can see, these statements remain valid.
> > 
> > Interesting; ;-)  What does these statement tells you ;-)  when applied
> > to a: and b: below?
> > 
> >   a: WRITE_ONCE(x, 1); // "preceding any prior RELEASE..."
> >   smp_store_release(&s, 1);
> >   smp_load_acquire(&s);
> >   b: WRITE_ONCE(y, 1); // "after an ACQUIRE..."
> 
> The first statement tells me that b: follows an ACQUIRE.
> 
> The second tells me that a: precedes a RELEASE.
> 
> And the third tells me that any READ_ONCE(x) statements coming po-after 
> b: would see x = 1 or a later value of x.  (Of course, they would have 
> to see that anyway because of the cache coherency rules.)

Mmh, something like "visible from the same CPU of the ACQUIRE" probably
could have helped me to reach the same conclusion.


> 
> More to the point, given:
> 
> P0()
> {
> 	WRITE_ONCE(x, 1);
> 	a: smp_store_release(&s, 1);
> }
> 
> P1()
> {
> 	b: r1 = smp_load_acquire(&s);
> 	r2 = READ_ONCE(x);
> }
> 
> the third statement tells me that if r1 = 1 (that is, if a: is prior to
> b:) then r2 must be 1.

Indeed; the "prior" is ambiguous, but yes.

  Andrea


> 
> Alan
>