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

Jonas Oberhauser
2023-01-24T15:12:04Z

On 1/24/2023 3:54 PM, Paul E. McKenney wrote:
> On Tue, Jan 24, 2023 at 12:09:48PM +0100, Andrea Parri wrote:
>>> There is the one below, but I am (1) not sure that I have it right,
>>> (2) not immediately certain that the Linux-kernel implementation would
>>> forbid it, (3) not immediately sure that it should be forbidden.
>>>
>>> In the meantime, thoughts?
>> As it stands, P0 to completion, then P1 to completion, then P2 to
>> completion should meet the "exists" clause; I guess we want "x=1"
>> in the clause (or the values of the stores to "x" exchanged).
> OK, so I still don't have it right. ;-)
>
> Make that x=1. I think.
>

If it is x=1, why doesn't LKMM forbid it?
Because T1:y=1 is read by T1 before the GP, the whole CS is before the
GP, i.e.,

srcu_read_unlock(s, r1); ->rcu-order synchronize_srcu(s);

The GP is furthermore po;prop;strong-fence;prop;po ordered before the
unlock, which you can shuffle around to get
Wx=2  ->prop;po;rcu-order;po ;  prop;strong-fence  Wx=2
or
Wx=2  ->rb  Wx=2
which is forbidden because rb is irreflexive.

Right?

jonas