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

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Date:   Mon, 5 Dec 2022 11:18:13 -0500
From:   "stern@...land.harvard.edu" <stern@...land.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@...wei.com>
Cc:     Boqun Feng <boqun.feng@...il.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 Mon, Dec 05, 2022 at 01:42:46PM +0000, Jonas Oberhauser wrote:
> > Besides, could you also explain a little bit why only "data;rfi" can be "carry-dep" but "ctrl;rfi" and "addr;rfi" cannot? I think it's because there are special cases when compilers can figure out a condition being true or an address being constant therefore break the dependency
> 
> Oh, good question. A bit hard for me to write down the answer clearly 
> (which some people will claim that I don't understand it well myself, 
> but I beg to differ :) :( :) ).
> 
> In a nutshell, it's because x ->data [Plain] ->rfi y ->... z fulfils 
> the same role as storing something in a register and then using it in 
> a subsequent computation; x ->ctrl y ->... z (and ->addr) don't. So 
> it's not due to smart compilers, just the fact that the other two 
> cases seem unrelated to the problem being solved, and including them 
> might introduce some unsoundness (not that I have checked if they do).

More can be said here.  Consider the following simple example:

	void P0(int *x, int *y)
	{
		int r1, r2;
		int a[10];

		r1 = READ_ONCE(*x);
		a[r1] = 1;
		r2 = a[r1];
		WRITE_ONCE(*y, r2);
	}

There is an address dependency from the READ_ONCE to the plain store in 
a[r1].  Then there is an rfi and a data dependency to the WRITE_ONCE.

But in this example, the WRITE_ONCE is _not_ ordered after the 
READ_ONCE, even though they are linked by (addr ; rfi ; data).  The 
compiler knows that the value of r1 does not change between the two 
plain accesses, so it knows that it can optimize the code to be:

	r1 = READ_ONCE(*x);
	r2 = 1;
	WRITE_ONCE(*y, r2);
	a[r1] = r2;

And then the CPU can execute the WRITE_ONCE before the READ_ONCE.  This 
shows that (addr ; rfi) must not be included in the carry-deps relation.

You may be able to come up with a similar argument for (ctrl ; rfi), 
although it might not be quite as clear.

Alan