Message-Id: <F9BC91E3-5B01-4BDE-B0C0-4A567473B1D2@in.tum.de>
Date:   Tue, 12 Jul 2022 08:45:51 +0200
From:   Paul Heidekrüger <Paul.Heidekrueger@...tum.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Marco Elver <elver@...gle.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Charalampos Mainas <charalampos.mainas@...il.com>,
        Pramod Bhatotia <pramod.bhatotia@...tum.de>,
        Soham Chakraborty <s.s.chakraborty@...elft.nl>,
        Martin Fink <martin.fink@...tum.de>
Subject: Re: [PATCH v2] tools/memory-model: Clarify LKMM's limitations in
 litmus-tests.txt

Paul E. McKenney <paulmck@...nel.org> wrote:

> On Mon, Jul 11, 2022 at 05:14:55PM +0200, Paul Heidekrüger wrote:
>>> On 8. Jul 2022, at 20:47, Paul E. McKenney <paulmck@...nel.org> wrote:
>>> 
>>> On Fri, Jul 08, 2022 at 10:45:06AM -0400, Alan Stern wrote:
>>>> On Fri, Jul 08, 2022 at 01:44:06PM +0200, Marco Elver wrote:
>>>>> On Tue, 14 Jun 2022 at 17:49, Paul Heidekrüger
>>>>> <paul.heidekrueger@...tum.de> wrote:
>>>>>> As discussed, clarify LKMM not recognizing certain kinds of orderings.
>>>>>> In particular, highlight the fact that LKMM might deliberately make
>>>>>> weaker guarantees than compilers and architectures.
>>>>>> 
>>>>>> Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@ethstick13.dse.in.tum.de/T/#u
>>>>>> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>
>>>>>> Co-developed-by: Alan Stern <stern@...land.harvard.edu>
>>>>> 
>>>>> Reviewed-by: Marco Elver <elver@...gle.com>
>>>>> 
>>>>> However with the Co-developed-by, this is missing Alan's SOB.
>>>> 
>>>> For the record:
>>>> 
>>>> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
>>>> 
>>>> (Note that according to Documentation/process/submitting-patches.rst, 
>>>> the submitting author's SOB is supposed to come last.)
>>> 
>>> And this is what I ended up with. Please provide additional feedback
>>> as needed, and in the meantime, thank you all!
>>> 
>>> 							Thanx, Paul
>> 
>> Looks great - my first commit in the Linux kernel!
> 
> Congratulations!!! ;-)

Thanks! Hopefully many more to come :-)

> My commits for the upcoming merge window, which is probably 2-3 weeks
> from now, are already set. So this is targeted at the merge window
> after that, which is likely to be in late September or early October.
> 
> So it is well on its way!

Awesome!

Many thanks,
Paul

> 							Thanx, Paul
> 
>> Thanks everyone!
>> 
>> Paul
>> 
>>> ------------------------------------------------------------------------
>>> 
>>> commit 3c7753e959706f39e1ee183ef8dcde3b4cfbb4c7
>>> Author: Paul Heidekrüger <paul.heidekrueger@...tum.de>
>>> Date: Tue Jun 14 15:48:11 2022 +0000
>>> 
>>> tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt
>>> 
>>> As discussed, clarify LKMM not recognizing certain kinds of orderings.
>>> In particular, highlight the fact that LKMM might deliberately make
>>> weaker guarantees than compilers and architectures.
>>> 
>>> Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@ethstick13.dse.in.tum.de/T/#u
>>> Co-developed-by: Alan Stern <stern@...land.harvard.edu>
>>> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
>>> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>
>>> Reviewed-by: Marco Elver <elver@...gle.com>
>>> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>>> Cc: Charalampos Mainas <charalampos.mainas@...il.com>
>>> Cc: Pramod Bhatotia <pramod.bhatotia@...tum.de>
>>> Cc: Soham Chakraborty <s.s.chakraborty@...elft.nl>
>>> Cc: Martin Fink <martin.fink@...tum.de>
>>> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>>> 
>>> diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt
>>> index 8a9d5d2787f9e..cc355999815cb 100644
>>> --- a/tools/memory-model/Documentation/litmus-tests.txt
>>> +++ b/tools/memory-model/Documentation/litmus-tests.txt
>>> @@ -946,22 +946,39 @@ Limitations of the Linux-kernel memory model (LKMM) include:
>>> 	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:
>>> +	Conversely, LKMM will sometimes overestimate the amount of
>>> +	reordering compilers and CPUs can carry out, leading it to miss
>>> +	some pretty obvious cases of ordering. 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.)
>>> +	The WRITE_ONCE() does not depend on the READ_ONCE(), and as a
>>> +	result, LKMM does not claim ordering. However, even though no
>>> +	dependency is present, the WRITE_ONCE() will not be executed before
>>> +	the READ_ONCE(). There are two reasons for this:
>>> +
>>> + The presence of the smp_mb() in one of the branches
>>> + prevents the compiler from moving the WRITE_ONCE()
>>> + up before the "if" statement, since the compiler has
>>> + to assume that r1 will sometimes be 0 (but see the
>>> + comment below);
>>> +
>>> + CPUs do not execute stores before po-earlier conditional
>>> + branches, even in cases where the store occurs after the
>>> + two arms of the branch have recombined.
>>> +
>>> +	It is clear that it is not dangerous in the slightest for LKMM to
>>> +	make weaker guarantees than architectures. In fact, it is
>>> +	desirable, as it gives compilers room for making optimizations. 
>>> +	For instance, suppose that a 0 value in r1 would trigger undefined
>>> +	behavior elsewhere. Then a clever compiler might deduce that r1
>>> +	can never be 0 in the if condition. As a result, said clever
>>> +	compiler might deem it safe to optimize away the smp_mb(),
>>> +	eliminating the branch and any ordering an architecture would
>>> +	guarantee otherwise.
>>> 
>>> 2.	Multiple access sizes for a single variable are not supported,
>>> 	and neither are misaligned or partially overlapping accesses.

Powered by blists - more mailing lists