> On 21. Jun 2022, at 16:24, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 21, 2022 at 01:59:27PM +0200, Paul Heidekrüger wrote:
>> OK. So, LKMM limits the scope of control dependencies to its arm(s), hence
>> there is a control dependency from the last READ_ONCE() before the loop
>> exists to the WRITE_ONCE().
>>
>> But then what about the following:
>>
>>> int *x, *y;
>>>
>>> int foo()
>>> {
>>> /* More code */
>>>
>>> if(READ_ONCE(x))
>>> return 42;
>>>
>>> /* More code */
>>>
>>> WRITE_ONCE(y, 42);
>>>
>>> /* More code */
>>>
>>> return 0;
>>> }
>>
>> The READ_ONCE() determines whether the WRITE_ONCE() will be executed at all,
>> but the WRITE_ONCE() doesn't lie in the if condition's arm.
>
> So in this case the LKMM would not recognize that there's a control
> dependency, even though it clearly exists.
Oh, that's unfortunate.
Then I would still argue that the "at all" definition is misleading. This
time in the other direction as I had initially proposed though, as the above
example is a case where "at all" holds true, but LKMM doesn't cover it. Or
do you think that caveating this in litmus-tests.txt, e.g. via the patch we
had recently worked out [1], is enough?
>> However, by
>> "inverting" the if, we get the following equivalent code:
>>
>>> if(!READ_ONCE(x)) {
>>> /* More code */
>>>
>>> WRITE_ONCE(y, 42);
>>>
>>> /* More code */
>>>
>>> return 0;
>>> }
>>>
>>> return 42;
>>
>> Now, the WRITE_ONCE() is in the if's arm, and there is clearly a control
>> dependency.
>
> Correct.
>
>> Similar cases:
>>
>>> if(READ_ONCE())
>>> foo(); /* WRITE_ONCE() in foo() */
>>> return 42;
>>
>> or
>>
>>> if(READ_ONCE())
>>> goto foo; /* WRITE_ONCE() after foo */
>>> return 42;
>>
>> In both cases, the WRITE_ONCE() again isn't in the if's arm syntactically
>> speaking, but again, with rewriting, you can end up with a control
>> dependency; in the first case via inlining, in the second case by simply
>> copying the code after the "foo" marker.
>
> Again, correct. The LKMM isn't always consistent, and it behaves this
> way to try to avoid presuming too much about the optimizations that
> compilers may apply.
Many thanks,
Paul
--
[1]: https://lore.kernel.org/all/20220614154812.1870099-1-paul.heidekrueger@xxxxxxxxx/Attachment:
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