linux-kernel - Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po

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Date:   Sun, 29 Jan 2023 20:36:45 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
        Andrea Parri <parri.andrea@...il.com>, will@...nel.org,
        peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com, dlustig@...dia.com, joel@...lfernandes.org,
        urezki@...il.com, quic_neeraju@...cinc.com, frederic@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po

On Sun, Jan 29, 2023 at 09:39:17PM -0500, Alan Stern wrote:
> On Sun, Jan 29, 2023 at 11:19:32PM +0100, Jonas Oberhauser wrote:
> > I see now. Somehow I thought stores must execute in program order, but I
> > guess it doesn't make sense.
> > In that sense, W ->xbstar&int X always means W propagates to X's CPU before
> > X executes.
> 
> It also means any write that propagates to W's CPU before W executes 
> also propagates to X's CPU before X executes (because it's the same CPU 
> and W executes before X).
> 
> > > Ideally we would fix this by changing the definition of po-rel to:
> > > 
> > > 	[M] ; (xbstar & int) ; [Release]
> > > 
> > > (This is closely related to the use of (xbstar & int) in the definition
> > > of vis that you asked about.)
> > 
> > This misses the property of release stores that any po-earlier store must
> > also execute before the release store.
> 
> I should have written:
> 
> 	[M] ; (po | (xbstar & int)) ; [Release]
> 
> > Perhaps it could be changed to the old  po-rel | [M] ; (xbstar & int) ;
> > [Release] but then one could instead move this into the definition of
> > cumul-fence.
> > In fact you'd probably want this for all the propagation fences, so
> > cumul-fence and pb should be the right place.
> > 
> > > Unfortunately we can't do this, because
> > > po-rel has to be defined long before xbstar.
> > 
> > You could do it, by turning the relation into one massive recursive
> > definition.
> 
> Which would make pretty much the entire memory model one big recursion.  
> I do not want to do that.
> 
> > Thinking about what the options are:
> > 1) accept the difference and run with it by making it consistent inside the
> > axiomatic model
> > 2) fix it through the recursive definition, which seems to be quite ugly but
> > also consistent with the power operational model as far as I can tell
> > 3) weaken the operational model... somehow
> > 4) just ignore the anomaly
> > 5) ???
> > 
> > Currently my least favorite option is 4) since it seems a bit off that the
> > reasoning applies in one specific case of LKMM, more specifically the data
> > race definition which should be equivalent to "the order of the two races
> > isn't fixed", but here the order isn't fixed but it's a data race.
> > I think the patch happens to almost do 1) because the xbstar&int at the end
> > should already imply ordering through the prop&int <= hb rule.
> > What would remain is to also exclude rcu-fence somehow.
> 
> IMO 1) is the best choice.
> 
> Alan
> 
> PS: For the record, here's a simpler litmus test to illustrates the 
> failing.  The idea is that Wz=1 is reordered before the store-release, 
> so it ought to propagate before Wy=1.  The LKMM does not require this.

In PowerPC terms, would this be like having the Wz=1 being reorders
before the Wy=1, but not before the lwsync instruction preceding the
Wy=1 that made it be a release store?

If so, we might have to keep this quirk.

							Thanx, Paul

> C before-release
> 
> {}
> 
> P0(int *x, int *y, int *z)
> {
> 	int r1;
> 
> 	r1 = READ_ONCE(*x);
> 	smp_store_release(y, 1);
> 	WRITE_ONCE(*z, 1);
> }
> 
> P1(int *x, int *y, int *z)
> {
> 	int r2;
> 
> 	r2 = READ_ONCE(*z);
> 	WRITE_ONCE(*x, r2);
> }
> 
> P2(int *x, int *y, int *z)
> {
> 	int r3;
> 	int r4;
> 
> 	r3 = READ_ONCE(*y);
> 	smp_rmb();
> 	r4 = READ_ONCE(*z);
> }
> 
> exists (0:r1=1 /\ 2:r3=1 /\ 2:r4=0)