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

Jonas Oberhauser
2023-01-27T15:03:59Z

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.

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.

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)