linux-kernel - Re: [PATCH] tools: memory-model: Document that the LKMM can easily miss control dependencies

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Date:   Sun, 4 Oct 2020 17:07:47 -0400
From:   joel@...lfernandes.org
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>, parri.andrea@...il.com,
        will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, dlustig@...dia.com, viro@...iv.linux.org.uk,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH] tools: memory-model: Document that the LKMM can easily
 miss control dependencies

On Sat, Oct 03, 2020 at 09:40:22PM -0400, Alan Stern wrote:
> Add a small section to the litmus-tests.txt documentation file for
> the Linux Kernel Memory Model explaining that the memory model often
> fails to recognize certain control dependencies.
> 
> Suggested-by: Akira Yokosawa <akiyks@...il.com>
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel

> 
> ---
> 
>  tools/memory-model/Documentation/litmus-tests.txt |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> Index: usb-devel/tools/memory-model/Documentation/litmus-tests.txt
> ===================================================================
> --- usb-devel.orig/tools/memory-model/Documentation/litmus-tests.txt
> +++ usb-devel/tools/memory-model/Documentation/litmus-tests.txt
> @@ -946,6 +946,23 @@ Limitations of the Linux-kernel memory m
>  	carrying a dependency, then the compiler can break that dependency
>  	by substituting a constant of that value.
>  
> +	Conversely, LKMM sometimes doesn't recognize that a particular
> +	optimization is not allowed, and as a result, thinks that a
> +	dependency is not present (because the optimization would break it).
> +	The memory model misses some pretty obvious control dependencies
> +	because of this limitation.  A simple example is:
> +
> +		r1 = READ_ONCE(x);
> +		if (r1 == 0)
> +			smp_mb();
> +		WRITE_ONCE(y, 1);
> +
> +	There is a control dependency from the READ_ONCE to the WRITE_ONCE,
> +	even when r1 is nonzero, but LKMM doesn't realize this and thinks
> +	that the write may execute before the read if r1 != 0.  (Yes, that
> +	doesn't make sense if you think about it, but the memory model's
> +	intelligence is limited.)
> +
>  2.	Multiple access sizes for a single variable are not supported,
>  	and neither are misaligned or partially overlapping accesses.
>