[cpp-threads] seq_cst compare_exchange and store-load fencing

[Alexander Terekhov]

Abbreviating too much...

All .store(...) below meant to store something nonzero (X and Y are
zero initially).

On Sat, Jan 3, 2009 at 3:07 PM, Alexander Terekhov
<alexander.terekhov at gmail.com> wrote:
> On Sat, Jan 3, 2009 at 2:09 AM, Paul E. McKenney
> <paulmck at linux.vnet.ibm.com> wrote:
>
> [... additional 1.10 notes about stunning sec_cst ...]
>
>> Works for me!
>
> Finally, I suggest to add rather simple example or two along the lines of
>
> P1: X.store(relaxed|release); fence(sec_cst); y = Y.load(relaxed|acquire);
> P2: Y.store(relaxed|release); fence(sec_cst); x = X.load(relaxed|acquire);

P1: X.store(1, relaxed|release); fence(sec_cst); y = Y.load(relaxed|acquire);
P2: Y.store(1, relaxed|release); fence(sec_cst); x = X.load(relaxed|acquire);

>
> (y=0, x=0 outcome is not allowed)
>
> stressing sec_cst stunning feature of NOT allowing to safely express
> the above in terms of
>
> P1: X.store(sec_cst); y = Y.load(relaxed|acquire);
> P2: Y.store(sec_cst); x = X.load(relaxed|acquire);
>

P1: X.store(1, sec_cst); y = Y.load(relaxed|acquire);
P2: Y.store(1, sec_cst); x = X.load(relaxed|acquire);

> (y=0, x=0 outcome is allowed)
>
> or
>
> P1: X.store(relaxed|release); y = Y.load(sec_cst);
> P2: Y.store(relaxed|release); x = X.load(sec_cst);

P1: X.store(1, relaxed|release); y = Y.load(sec_cst);
P2: Y.store(1, relaxed|release); x = X.load(sec_cst);

>
> (y=0, x=0 outcome is allowed)

or

P1: X.store(1, sec_cst); y = Y.load(relaxed|acquire);
P2: Y.store(1, relaxed|release); x = X.load(sec_cst);

(y=0, x=0 outcome is allowed)

or

P1: X.store(1, relaxed|release); y = Y.load(sec_cst);
P2: Y.store(1, sec_cst); x = X.load(relaxed|acquire);

(y=0, x=0 outcome is allowed)

or

<combinations with one sec_cst fence and one sec_cst load/store (not
fence) by another P>

(y=0, x=0 outcome is allowed)

>
> right?
>
> Wow.

And once again: Wow.

regards,
alexander.