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

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Message-ID: <20190226104551.GF32534@hirez.programming.kicks-ass.net>
Date:   Tue, 26 Feb 2019 11:45:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     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 10:30:09AM +0100, Peter Zijlstra wrote:
> On Mon, Feb 25, 2019 at 09:55:17AM -0800, Paul E. McKenney wrote:
> > But if you know of any code in the Linux kernel that needs to compare
> > pointers, one of which might be in the process of being freed, please
> > do point me at it.
> 
> I'm having the utmost difficulty of understanding why that would be a
> problem. It's just a value. Freeing memory does not affect the actual
> memory or any pointers to it.
> 
> *confusion*
> 
> None of this makes any kind of sense.

I found and started to read:

  www.open-std.org/jtc1/sc22/wg14/www/docs/n2311.pdf

and that's all massive bong-hits. That's utterly insane.

Even the proposed semantics are crazy; they include UB and are therefore
broken on principle. But also; the provenance ID has words like:
"allocated storage duration", how is that well defined in the context of
concurrent execution?

Also, isn't the kernel filled with inter-object pointer arithmetic?

PNVI also looks like something that simply cannot work; how are we at
compile time supposed to know the active (concurrent modified) heap
layout. What is a 'live' object?