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

[Jonas Oberhauser]

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