Re: [RFC] tools/memory-model: Rule out OOTA

On Mon, Jan 13, 2025 at 02:04:58PM -0800, Paul E. McKenney wrote:
> On Fri, Jan 10, 2025 at 05:21:59PM +0100, Jonas Oberhauser wrote:
> > Am 1/10/2025 um 3:54 PM schrieb Paul E. McKenney:
> > > On Thu, Jan 09, 2025 at 07:35:19PM +0100, Jonas Oberhauser wrote:
> > > > Am 1/9/2025 um 6:54 PM schrieb Paul E. McKenney:
> > > > > On Wed, Jan 08, 2025 at 08:17:51PM +0100, Jonas Oberhauser wrote:

[ . . . ]

> > > > I currently can not come up with an example where there would be a
> > > > (semantic) control dependency from a load to a store that is not in the arm
> > > > of an if statement (or a loop / switch of some form with the branch
> > > > depending on the load).
> > > > 
> > > > I think the control dependency is just a red herring. It is only there to
> > > > avoid the data race.
> > > 
> > > Well, that red herring needs to have a companion fish to swim with in
> > > order to enforce ordering, and I am not seeing that companion.
> > > 
> > > Or am I (yet again!) missing something subtle here?
> > 
> > It makes more sense to think about how people do message passing (or
> > seqlock), which might look something like this:
> > 
> >   [READ_ONCE]
> >   rmb
> >   [plain read]
> > 
> > and
> > 
> >   [plain write]
> >   wmb
> >   [WRITE_ONCE]
> > 
> > 
> > Clearly LKMM says that there is some sort of order (not quite happens-before
> > order though) between the READ_ONCE and the plain read, and between the
> > plain write and the WRITE_ONCE. This order is clearly defined in the data
> > race definition, in r-pre-bounded and w-post-bounded.
> > 
> > Now consider
> > 
> >   [READ_ONCE]
> >   rmb
> >   [plain read]
> >   // some code that creates order between the plain accesses
> >   [plain write]
> >   wmb
> >   [WRITE_ONCE]
> > 
> > where for some specific reason we can discern that the compiler can not
> > fully eliminate/move across the barrier either this specific plain read, nor
> > the plain write, nor the ordering between the two.
> > 
> > In this case, is there order between the READ_ONCE and the WRITE_ONCE, or
> > not? Of course, we know current LKMM says no. I would say that in those very
> > specific cases, we do have ordering.
> 
> Agreed, for LKMM to deal with seqlock, the read-side critical section
> would need to use READ_ONCE(), which is a bit unnatural.  The C++
> standards committee has been discussing this for some time, as that
> memory model also gives data race in that case.
> 
> But it might be better to directly model seqlock than to try to make
> LKMM deal with the underlying atomic operations.

Maybe I should give an example approach, perhaps inspiring a better
approach.

o	Model reader-writer locks in LKMM, including a relaxed
	write-lock-held primitive.

o	Model sequence locks in terms of reader-writer locks:

	o	The seqlock writer maps to write lock.

	o	The seqlock reader maps to read lock, but with a
		write-lock-held check.	If the write lock is held at
		that point, the seqlock tells the caller to retry.

		Please note that the point is simply to exercise
		the failure path.

	o	If a value is read in the seqlock reader and used
		across a "you need to retry" indication, that
		flags a seqlock data race.

But is there a better way?

						Thanx, Paul