Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po

[Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>]

On 2/26/2023 4:30 AM, Alan Stern wrote:
> On Sat, Feb 25, 2023 at 07:09:05PM -0800, Boqun Feng wrote:
>> On Sat, Feb 25, 2023 at 09:29:51PM -0500, Alan Stern wrote:
>>> On Sat, Feb 25, 2023 at 05:01:10PM -0800, Paul E. McKenney wrote:
>>>> A few other oddities:
>>>>
>>>> litmus/auto/C-LB-Lww+R-OC.litmus
>>>>
>>>> 	Both versions flag a data race, which I am not seeing.	It appears
>>>> 	to me that P1's store to u0 cannot happen unless P0's store
>>>> 	has completed.  So what am I missing here?
>>> The LKMM doesn't believe that a control or data dependency orders a
>>> plain write after a marked read.  Hence in this test it thinks that P1's
>>> store to u0 can happen before the load of x1.  I don't remember why we
>>> did it this way -- probably we just wanted to minimize the restrictions
>>> on when plain accesses can execute.  (I do remember the reason for
>>> making address dependencies induce order; it was so RCU would work.)
>>>
>> Because plain store can be optimzed as an "store only if not equal"?
>> As the following sentenses in the explanations.txt:
>>
>> 	The need to distinguish between r- and w-bounding raises yet another
>> 	issue.  When the source code contains a plain store, the compiler is
>> 	allowed to put plain loads of the same location into the object code.
>> 	For example, given the source code:
>>
>> 		x = 1;
>>
>> 	the compiler is theoretically allowed to generate object code that
>> 	looks like:
>>
>> 		if (x != 1)
>> 			x = 1;
>>
>> 	thereby adding a load (and possibly replacing the store entirely).
>> 	For this reason, whenever the LKMM requires a plain store to be
>> 	w-pre-bounded or w-post-bounded by a marked access, it also requires
>> 	the store to be r-pre-bounded or r-post-bounded, so as to handle cases
>> 	where the compiler adds a load.
> Good guess; maybe that was the reason.  [...]
> So perhaps the original reason is not valid now
> that the memory model explicitly includes tests for stores being
> r-pre/post-bounded.
>
> Alan

I agree, I think you could relax that condition.

Note there's also rw-xbstar (used with fr) which doesn't check for
r-pre-bounded, but it should be ok. That's because only reads would be
unordered, as a result the read (in the if (x != ..) x=..) should provide
the correct value. The store would be issued as necessary, and the issued
store would still be ordered correctly w.r.t the read.

Best wishes,
jonas