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

[Alexander Terekhov]

On Fri, Jan 2, 2009 at 9:07 PM, Paul E. McKenney
<paulmck at linux.vnet.ibm.com> wrote:
[...]
>> Formalities regarding distinguishing
>>
>> atomic_thread_fence(seq_cst), X.load(relaxed|acquire);
>>
>> vs.
>>
>> X.load(seq_cst);
>>

Subsequent "aside for a moment..." phrase managed to escape here.
Sorry, got interrupted. ;-)

>> why not state it in a more prominent place like 1.10 instead of 1K
>> pages below it? ;-)
>
> Because you have to go 1K pages below it even to find out about the
> memory_order enumeration.  ;-)

Note that 1.10 in N2800 versison already does mention both
"sequentially consistent" and "sequential consistency".

>
>> > Now I personally have no objection to making seq_cst operations more
>> > expensive, but others might.  ;-)
>>
>> I suspect that not making seq_cst operations more expensive (according
>> to simple "fully-fenced" reasoning) will result in quite a lot of
>> incorrect code.
>>
>> We'll see.
>
> Pretty simple rule: "If you want seq_cst operations to be guaranteed
> ordered WRT non-seq_cst operations, use atomic_thread_fence(seq_cst)
> to segregate the operations."

Okay, so why not add to C++ 1.10 (as in C++ 1.10 according to the
N2800 version) additional paragraph 17:

"[/Note:/ Nothing requires that sequentially consistent operations
[seq_cst, see 29.1] be ordered with respect to non-sequentially
consistent operations [atomics as well, see 29.1] except as required
by acquire, consume, and release semantics [see 29.1]. -- /end note/"

-and-

18:

"[/Note:/ If you want sequentially consistent [seq_cst, see 29.1]
results, use sequential consistency [seq_cst, see 29.1] everywhere.
On the other hand, if you want to use optimizations involving
non-sequentially consistent atomics [!seq_cst, see 29.1], make sure
that you understand the actual guarantees that sequentially consistent
[seq_cst, see 29.1] provide. -- /end note/]

-and-

19:

"[/Note:/ If you want sequentially consistent [seq_cst, see 29.1]
operations to be guaranteed ordered WRT non-seq_cst operations
[!seq_cst, see 29.1], use atomic_thread_fence(seq_cst) [see 29.6] to
segregate the operations. -- /end note/"

:-)

regards,
alexander.