linux-kernel - Re: [PATCH v2] tools: memory-model: Make plain accesses carry dependencies

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Date:   Thu, 8 Dec 2022 16:37:35 -0500
From:   "stern@...land.harvard.edu" <stern@...land.harvard.edu>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Jonas Oberhauser <jonas.oberhauser@...wei.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>,
        "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 v2] tools: memory-model: Make plain accesses carry
 dependencies

On Tue, Dec 06, 2022 at 12:52:47PM -0800, Boqun Feng wrote:
> On Tue, Dec 06, 2022 at 12:46:58PM -0800, Boqun Feng wrote:
> > Thank you, Alan! One question though, can a "smart" compiler optimize
> > out the case below, with the same logic?
> > 
> > 	void P0(int *x, int *y, int *a)
> > 	{
> > 		int r1, r2;
> > 
> > 		r1 = READ_ONCE(*x); // A
> > 
> > 		*a = r1 & 0xffff; // B
> > 
> > 		r2 = *a & 0xffff0000; // C
> > 
> > 		WRITE_ONCE(*y, r2); // D
> > 
> > 	}
> > 
> > I think we have A ->data B ->rfi C ->data D, however a "smart" compiler
> > can figure out that r2 is actually zero, right? And the code get
> > optimized to:
> > 
> > 	r1 = READ_ONCE(*x);
> > 	r2 = 0;
> > 	WRITE_ONCE(*y, r2);
> > 	*a = r1 & 0xffff;
> > 
> > and break the dependency.

Yes, that could happen.

> > I know that our memory model is actually unware of the differences of
> > syntatics dependencies vs semantics syntatics, so one may argue that in
> > the (data; rfi) example above the compiler optimization is outside the
> > scope of LKMM, but won't the same reasoning apply to the (addr; rfi)
> > example from you? The WRITE_ONCE() _syntatically_ depends on load of
> > a[r1], therefore even a "smart" compiler can figure out the value, LKMM
> 
> I guess it should be that r2 (i.e. the load of a[r1]) _syntatically_
> depends on the value of r1.

Yes.  But more to the point, the LKMM already has this problem for 
ordinary dependencies.  If you do:

	r1 = READ_ONCE(*x);
	r2 = r1 & 0x0000ffff;
	r3 = r2 & 0xffff0000;
	WRITE_ONCE(*y, r3);

then the LKMM will think there is a dependency (because there is a 
_syntactic_ dependency), but the compiler is likely to realize that 
there isn't a _semantic_ dependency and will destroy the ordering.

We warn people about this possibility, and the same warning applies to 
dependencies carried by plain accesses.  So I don't think this is a 
reason to object to Jonas's carries-dep relation.

Alan