linux-kernel - Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo

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Date:   Tue, 26 Feb 2019 16:04:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>
Subject: Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo

On Tue, Feb 26, 2019 at 06:28:45AM -0800, Paul E. McKenney wrote:

> Yes, this all is a bit on the insane side from a kernel viewpoint.
> But the paper you found does not impose this; it has instead been there
> for about 20 years, back before C and C++ admitted to the existence
> of concurrency.  But of course compilers are getting more aggressive,
> and yes, some of the problems show up in single-threaded code.

But that paper is from last year!! It has Peter Sewell on, I'm sure he's
heard of concurrency.

> The usual response is "then cast the pointers to intptr_t!" but of
> course that breaks type checking.

I tried laundering the pointer through intptr_t, but I can't seem to
unbreak it.


root@...-ep:~/tmp# gcc-8 -O2 -fno-strict-aliasing  -o ptr ptr.c ; ./ptr
p=0x55aacdc80034 q=0x55aacdc80034
x=1 y=2 *p=11 *q=2
root@...-ep:~/tmp# cat ptr.c
#include <stdio.h>
#include <string.h>
#include <stdint.h>
int y = 2, x = 1;
int main (int argc, char **argv) {
	intptr_t P = (intptr_t)&x;
	intptr_t Q = (intptr_t)&y;
	P += sizeof(int);
	int *q = &y;
	printf("p=%p q=%p\n", (int*)P, (int*)Q);
	if (P == Q) {
		int *p = (int *)P;
		*p = 11;
		printf("x=%d y=%d *p=%d *q=%d\n", x, y, *p, *q);
	}
}