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Groups > comp.programming.threads > #284
| From | Edek <edek.pienkowski@gmail.com> |
|---|---|
| Newsgroups | comp.programming.threads |
| Subject | Re: questions about memory_order_seq_cst fence |
| Date | 2011-06-16 00:21 +0200 |
| Organization | ATMAN - ATM S.A. |
| Message-ID | <itbb7k$uhh$1@node2.news.atman.pl> (permalink) |
| References | <43eb372b-6517-4914-b647-860f1f7136ba@k15g2000pri.googlegroups.com> <it5sht$i7j$1@node2.news.atman.pl> <b7657dae-d49f-4859-8020-b5d76bfae2cd@p9g2000prh.googlegroups.com> |
On 06/14/2011 05:27 PM, Masakuni Oishi wrote:
> Edek<edek.pienkow...@gmail.com> wrote:
>> I'm tired and I haven't followed the C++11 discussion,
>> but are the fences relevant to the atomic operations
>> themselves? I mean, aren't they for enforcing regular
>> variables around atomics?
>
> The fences enforce ordering for not only regular variables
> but also atomic variables.
> For example:
>
> // Initially
> atomic<int> x(0), y(0);
>
> // Thread 1:
> y.store(1, memory_order_relaxed); <--- this needn't be atomic,
it just happens to be
> x.store(1, memory_order_release); <---- This is like releasing a lock
so y store must be done earlier
>
> // Thread 2:
> r1 = x.load(memory_order_acquire); <--- This is like acquiring a lock
so y load must be done later
> r2 = y.load(memory_order_relaxed); <--- Might not be atomic
>
>
> In this code, if r1 is 1, then r2 must be 1, not 0.
>
> -- masakuni
Yes, that is what I actually meant (sorry, I'm a plain novice in this
weird atomic stuff). See my above small comments.
But what I also meant is the following, simplifying things
> // Thread 2:
> x.store(1, memory_order_release);
This seems not to enforce anything, if there is no code above it
in thread 2. Does it enforce anything (apart from atomicity
of write)?
(Quite antique these days) quote, from n2427
memory_order_release
Performs a release operation on the affected
memory locations, thus
making regular memory writes visible to other
threads through the atomic variable to which it is applied.
This, however, is a sequence point (interacts with thread 1's and
thread 2's fences):
> // Thread 2:
> x.store(1, memory_order_seq_cst); // (3)
Quote:
...in addition, has sequentially-consistent operation ordering.
So, if I understand this mail thread correctly, the question is whether
in the code 1 case from your question the change to x made by thread 2,
but - to my understanding - not really ordered in any way in code 1,
is visible by threads 1 and 3 in a specified order enforced by these
threads' (1 & 3) fences, or not necessarily (and y is just to show the
constraints - if we can look at it for a moment from this aspect).
Is everything above right, or is it more subtle?
Edek
PS.
As a side note, I compiled all ops on x86_64, and atomic<int>::load
is mfence/load/mfence even for relaxed. I'd need some time to analyse
the thing from perspective of other ops implementations in all
orderings, and this leads me to the not so subtle question:
I understand people _should definitely_ use the default.
I assume the goal, or the official word for it is 'rationale', was
to let people 'who understand' write code that would be both portable
and more efficient than the default. I am somehow beginning to doubt
both my capabilities and whether this will be at all possible - I mean
the efficiency, not portability (the latter can be simply done right).
Though maybe the bottom line is that at least the precise implementation
will have at least a couple of fences less than the default, at the
price of - I think no one argues about that - complexity.
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questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-13 09:40 -0700
Re: questions about memory_order_seq_cst fence Edek <edek.pienkowski@gmail.com> - 2011-06-13 22:39 +0200
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-14 08:27 -0700
Re: questions about memory_order_seq_cst fence Edek <edek.pienkowski@gmail.com> - 2011-06-16 00:21 +0200
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-15 15:48 -0700
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-16 05:03 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 08:35 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-16 12:05 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 11:19 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-16 13:57 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 13:51 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-16 16:53 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 16:17 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-16 17:37 +0200
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 08:47 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 16:54 +0100
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 09:01 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 17:07 +0100
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 13:59 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-16 14:32 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-17 08:03 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-17 16:19 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-17 15:40 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-30 14:26 +0200
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-17 15:24 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-18 03:49 -0700
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-18 14:40 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-18 15:46 -0700
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-18 20:27 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-18 21:31 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-18 21:37 -0700
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-20 15:45 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-20 20:05 -0700
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-18 14:06 +0200
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-16 18:32 +0200
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 15:42 -0700
Re: questions about memory_order_seq_cst fence Edek <edek.pienkowski@gmail.com> - 2011-07-10 19:47 +0200
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 08:51 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 17:23 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-16 18:49 +0200
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 08:51 -0700
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-16 17:25 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 16:56 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-18 14:28 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-20 09:54 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-20 15:39 +0200
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-22 13:24 +0200
Re: questions about memory_order_seq_cst fence Edek <edek.pienkowski@gmail.com> - 2011-06-16 14:39 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 13:55 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-16 12:03 +0200
Re: questions about memory_order_seq_cst fence Edek <edek.pienkowski@gmail.com> - 2011-07-10 19:10 +0200
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-13 16:00 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-13 23:31 -0700
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-14 08:32 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-14 23:05 +0100
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-14 16:44 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-15 08:34 +0100
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-15 02:11 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-15 15:58 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 08:26 +0100
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-16 05:05 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-16 13:11 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-18 15:42 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-20 09:27 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-20 15:30 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-20 15:00 +0100
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-21 14:54 +0200
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-21 18:37 +0200
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-21 15:59 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-22 09:01 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-20 15:41 +0100
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-20 23:24 -0700
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-22 10:46 -0700
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-22 11:19 -0700
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-22 11:57 -0700
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-27 14:05 +0200
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-29 16:14 -0700
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-06-24 01:42 -0700
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-06-27 13:24 +0200
Re: questions about memory_order_seq_cst fence Anthony Williams <anthony.ajw@gmail.com> - 2011-07-01 02:25 -0700
Re: questions about memory_order_seq_cst fence Alexander Terekhov <terekhov@web.de> - 2011-07-01 12:48 +0200
Re: questions about memory_order_seq_cst fence Masakuni Oishi <yamasa@bsdhouse.org> - 2011-06-15 01:53 -0700
Re: questions about memory_order_seq_cst fence Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-15 02:57 -0700
Re: questions about memory_order_seq_cst fence "Chris M. Thomasson" <cristom@charter.net> - 2011-06-15 02:06 -0700
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