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Re: questions about memory_order_seq_cst fence

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From Anthony Williams <anthony.ajw@gmail.com>
Newsgroups comp.programming.threads
Subject Re: questions about memory_order_seq_cst fence
References <43eb372b-6517-4914-b647-860f1f7136ba@k15g2000pri.googlegroups.com> <87tybsm5kq.fsf@justsoftwaresolutions.co.uk> <35ec803e-2871-41ce-97ea-8b615b29050f@r33g2000prh.googlegroups.com> <87hb7r5z03.fsf@justsoftwaresolutions.co.uk>
Date Thu, 16 Jun 2011 08:26:16 +0100
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Anthony Williams <anthony.ajw@gmail.com> writes:

> Joshua Maurice <joshuamaurice@gmail.com> writes:
>
>>> 29.3p7 of N3290 says:
>>>
>>> "For atomic operations A and B on an atomic object M, if there are
>>> memory_order_seq_cst fences X and Y such that A is sequenced before X, Y
>>> is sequenced before B, and X precedes Y in S, then B occurs later than A
>>> in the modification order of M."
>>>
>> I think it's meant to read:
>> []
>> For atomic **write (and atomic read-modify-write operations)** A and B
>> on an atomic object M, if there are memory_order_seq_cst fences X and
>> Y such that A is sequenced before X, Y is sequenced before B, and X
>> precedes Y in S, then B occurs later than A in the modification order
>> of M.
>> [/]
>
> When we drafted this words, I think we meant "operation". It is the
> reference to "modification order" that is the typo. What is meant is
> that if A is a write and B is a read then B must read the value written
> by A, or something later in the modification order; if A is a write and
> B is a write then B must be later than A in the MO; if A is a read and B
> is a write then A must read a value prior to B in the MO, and if A is a
> read and B is a read then B must read the same value as A or a value
> later in the MO.

> After rereading this section of the standard several times, I'm no
> longer sure. I'll have to check my notes.

I've checked my notes. My recollection was wrong.

We *discussed* requiring seq_cst fences to order loads like this, but
decided to explicitly limit the orderings to writes. The approved
wording said "where A and B modify M", but this somehow got lost.

In particular, it was felt that requiring this would add complexity to
the IA-64 implementation of fences, and potentially require that relaxed
operations on that architecture would also have to be more expensive,
thus requiring people to pay for features they didn't need.

Anthony
-- 
Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
just::thread C++0x thread library             http://www.stdthread.co.uk
Just Software Solutions Ltd       http://www.justsoftwaresolutions.co.uk
15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976

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Thread

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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