Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

[Paul E. McKenney]

On Fri, Jan 27, 2023 at 04:03:16PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/26/2023 7:48 PM, Paul E. McKenney wrote:
> > On Thu, Jan 26, 2023 at 01:17:49PM +0100, Jonas Oberhauser wrote:
> > > [...]
> > > Note that this interpretation is analogous to the promise of smp_mb__after_unlock_lock(), which says that an
> > > UNLOCK+LOCK pair act as a full fence: here the read-side unlock+gp act as a
> > > full memory barrier.
> > Good point that the existing smp_mb__after_unlock_lock() can be used for
> > any use cases relying on the more literal interpretation of this promise.
> > We already have the work-around!  ;-)
> 
> Can it? I meant that the less-literal form is similar to the one given by
> smp_mb__after_unlock_lock().
> 
> > > [...] I suppose you might be able to write
> > > some absurd client that inspects every store of the reader thread and sees
> > > that there is no line in the reader side code that acts like a full fence.
> > > But it would take a lot of effort to discern this.
> > The usual litmus test is shown at the end of this email [...]
> > > [...] I hope few people would have this unhealthy idea. But you
> > > never know.
> > Given that the more literal interpretation is not unreasonable, we should
> > assume that someone somewhere might have interpreted it that way.
> > 
> > But I agree that the odds of someone actually relying on this are low,
> > and any such use case can be fixed with smp_mb__before_srcu_read_unlock(),
> > similar to smp_mb__after_srcu_read_unlock() that you note is already in use.
> > 
> > It would still be good to scan SRCU use for this sort of pattern, maybe
> > manually, maybe via something like coccinelle.  Alternatively, I could
> > post on my blog (with right of first refusal to LWN and you guys as
> > co-authors) telling the community of our intent to change this and see
> > what people say.  Probably both rather than either/or.
> > 
> > Thoughts?
> 
> My first thought is "there is a 'usual' litmus test for this?" :D
> But yes, the test you have given has at least the same structure as what I
> would expect.

Exactly!  ;-)

> Communicating this with the community sounds very reasonable.
> 
> For some automated combing, I'm really not sure what pattern to look for.
> I'm afraid someone with a lot of time might have to look (semi-)manually.

Please continue giving it some thought.  The number of srcu_read_unlock()
calls in v6.1 is about 250, which is within the realm of manual
inspection, but it is all too easy to miss something.

							Thanx, Paul

> Best wishes, jonas
> 
> 
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > C C-srcu-observed-6
> > 
> > (*
> >   * Result: Sometimes
> >   *
> >   * The result is Never if any of the smp_mb() calls is uncommented.
> >   *)
> > 
> > {}
> > 
> > P0(int *a, int *b, int *c, int *d, struct srcu_struct *s)
> > {
> > 	int r1;
> > 	int r2;
> > 	int r3;
> > 	int r4;
> > 
> > 	r1 = srcu_read_lock(s);
> > 	WRITE_ONCE(*b, 2);
> > 	r2 = READ_ONCE(*a);
> > 	// smp_mb();
> > 	srcu_read_unlock(s, r1);
> > 	// smp_mb();
> > 	r3 = READ_ONCE(*c);
> > 	// smp_mb();
> > 	r4 = READ_ONCE(*d);
> > }
> > 
> > P1(int *a, int *b, int *c, int *d, struct srcu_struct *s)
> > {
> > 	WRITE_ONCE(*b, 1);
> > 	synchronize_srcu(s);
> > 	WRITE_ONCE(*c, 1);
> > }
> > 
> > P2(int *a, int *b, int *c, int *d, struct srcu_struct *s)
> > {
> > 	WRITE_ONCE(*d, 1);
> > 	smp_mb();
> > 	WRITE_ONCE(*a, 1);
> > }
> > 
> > exists (0:r2=1 /\ 0:r3=1 /\ 0:r4=0 /\ b=1)
>