Re: [PATCH RFC] tools/memory-model: Adjust ctrl dependency definition

[Paul Heidekrüger]

Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, Jun 27, 2022 at 11:47:43AM +0200, Paul Heidekrüger wrote:
>>> 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
> 
> I agree, and I would welcome a patch improving the definition. Perhaps 
> something along the lines of what I wrote earlier in this email thread.

I have just been thinking about how to word this patch; am I correct in
assuming that the LKMM does not deal with loop conditions? Or in other
words, there is no way for a loop condition to impose a ctrl dependency on
any WRITE_ONCE's in the loop body? It are only if and switch statements the
LKMM is concerned with in the case of ctrl dependencies?

Many thanks,
Paul

>> 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?
> 
> No, the explanation should be improved.
> 
> Alan