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SubjectRe: [PATCH v2] tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt
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Paul E. McKenney <paulmck@kernel.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@kernel.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@in.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@in.tum.de>
>>>>>> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
>>>>>
>>>>> Reviewed-by: Marco Elver <elver@google.com>
>>>>>
>>>>> However with the Co-developed-by, this is missing Alan's SOB.
>>>>
>>>> For the record:
>>>>
>>>> Signed-off-by: Alan Stern <stern@rowland.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@in.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@rowland.harvard.edu>
>>> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
>>> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de>
>>> Reviewed-by: Marco Elver <elver@google.com>
>>> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>>> Cc: Charalampos Mainas <charalampos.mainas@gmail.com>
>>> Cc: Pramod Bhatotia <pramod.bhatotia@in.tum.de>
>>> Cc: Soham Chakraborty <s.s.chakraborty@tudelft.nl>
>>> Cc: Martin Fink <martin.fink@in.tum.de>
>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.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.


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