linux-kernel - Re: [PATCH] tools: memory-model: Make plain accesses carry dependencies

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Date:   Sat, 3 Dec 2022 10:20:28 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@...wei.com>
Cc:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "will@...nel.org" <will@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "urezki@...il.com" <urezki@...il.com>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tools: memory-model: Make plain accesses carry
 dependencies

On Sat, Dec 03, 2022 at 11:47:06AM +0000, Jonas Oberhauser wrote:
> 
> 
> -----Original Message-----
> From: Alan Stern [mailto:stern@...land.harvard.edu] 
> Sent: Friday, December 2, 2022 9:22 PM
> 
> > > void *y[2];
> > > void *x[2] = { (void*)&y[1], (void*)&y[0] };
> > > 
> > > P0() {
> > >     void **t = (void**)(x[0]);
> 
> > Now t holds a pointer to y[1].
> 
> Unfortunately, this kind of inductive reasoning (arguing about what happens based on what happened "before") is not possible with memory models that allow OOTA; as you put it, one must allow for loads reading from stores that haven't happened yet.
> One such store (I promise!(*)) is a store to x[0] which writes &x[1]. Let's consider the alternative universe where we read from this future store, so now t holds a pointer to x[1].
> 
> > >     *t = (void*)(t-1);
> 
> > And now y[1] holds a pointer to y[0].
> 
> In our alternative universe, x[1] now holds a pointer to x[0].
> 
> 
> > > }
> > > P1() {
> > >     void **u = (void**)(x[1]);	
> 
> > Now u holds a pointer to y[0].
> 
> In our alternative universe, u holds the pointer to x[0] stored by P0().
> 
> > >     *u = (void*)(u+1);
> 
> > And now y[0] holds a pointer to y[1].
> 
> In our alternative universe, now x[0] holds a pointer to x[1]. Behold, the store I promised would happen!
> 
> > > }
> 
> > The contents of x[] never get changed, so there's no question about the values of t and u.
> 
> They might get changed, by the stores *t=... and *u=...

Okay, now I understand.  Yes, this counts as an example of values 
appearing out of thin air, even though the values are constrained.  The 
same sort of thing can happen in a less exotic form, such as:

	P0		P1
	------		-----------
	x = y;		y = x & ~1;

Then in an OOTA execution, x and y can end up holding any _even_ value.  
Yes, constraining a value to be even is less drastic than constraining 
it to be &x[0] as in your example, but it's still a constraint.  One 
can easily extend this example in various directions.

Alan