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Re: std::atomic<T>::is_lock_free()

From "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups comp.lang.c++
Subject Re: std::atomic<T>::is_lock_free()
Date 2019-11-15 13:56 -0800
Organization Aioe.org NNTP Server
Message-ID <qqn6uj$1m73$1@gioia.aioe.org> (permalink)
References (8 earlier) <qqjvki$n35$4@news.albasani.net> <qqkhm9$1e2a$1@gioia.aioe.org> <qqlnp9$3ed$1@news.albasani.net> <qqn3ip$16u6$1@gioia.aioe.org> <b2FzF.150668$oT5.19859@fx47.iad>

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On 11/15/2019 1:47 PM, Scott Lurndal wrote:
> "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:
>> On 11/15/2019 12:31 AM, Bonita Montero wrote:
>>>>>> The proposal provides lock-free query functions on individual
>>>>>> objects rather than whole types to permit unavoidably misaligned
>>>>>> atomic variables without penalizing the performance of aligned
>>>>>> atomic variables.
>>>
>>>>> That's rather stupid since the language says that access to unaligned
>>>>> data has an undefined behaviour. And there's not the least necessitiy
>>>>> in using unaligned atomics.
>>>
>>>> You are JG, right?
>>>
>>> Tell me of the sense to use non-aligned atomics.
>>
>> Not sure what that even means. Are you talking about L2 cache alignment
>> and proper padding? Its used to avoid false sharing.
> 
> Why do you bother with BM?  It deliberatly removes attributes so
> it's impossible to see who said what to whom.

I think BM = JG. Its a sock puppet...

> In any case, unaligned atomics are useless, and many architectures
> won't support them (particularly in uncachable memory regions like
> those mapped to PCI express bars (and PCI express endpoints -may- support atomic
> operations...)).
> 
> I suspect that most architectures avoid supporting unaligned atomics
> because they might split cache lines which would make it impossible
> (or very time consuming) to support atomic semantics for the access.
> 

Well, I remember abusing unaligned atomics such that a word straddles a 
L2 cache line! Well, on x86 this should trigger a system wide bus lock 
for legacy reasons. It did something quite interesting. It acted like a 
remote memory barrier that makes it possible to implement asymmetric 
sync algorithms. There is a mention of it, need to find it. They called 
it something like QPI? Humm... Let me do a little search...

[searching]

Found it!

https://blogs.oracle.com/dave/qpi-quiescence

This is very hackish, but does allow for RCU. Also, fwiw, Windows has an 
interesting instruction:

https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-flushprocesswritebuffers

That can provide a similar behavior. Iirc, it sends IPIs to an affinity 
mask of processors. This can be done in the Kernel with Windows. It 
takes a device driver.

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std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 10:54 +0100
  Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 11:24 +0100
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 14:11 +0100
      Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 14:39 +0100
        Re: std::atomic<T>::is_lock_free() Bo Persson <bo@bo-persson.se> - 2019-11-12 16:46 +0100
          Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 17:40 +0100
            Re: std::atomic<T>::is_lock_free() Bo Persson <bo@bo-persson.se> - 2019-11-12 20:23 +0100
              Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-12 15:21 -0800
              Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:09 -0800
              Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-13 06:34 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:00 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:26 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:33 -0800
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:33 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:51 +0100
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:50 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 23:14 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 10:43 +0100
  Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-12 15:14 -0800
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:11 +0100
  Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 07:53 -0800
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 16:57 +0100
      Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 08:16 -0800
        Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:26 +0100
          Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:28 +0100
          Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 08:30 -0800
            Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:33 +0100
              Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:41 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 09:31 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 12:59 -0800
                Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-15 21:47 +0000
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:56 -0800
                Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-16 18:07 +0000
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-16 13:55 -0800
                Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-16 22:13 +0000
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-16 18:40 -0800
                Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-17 18:39 -0500
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 20:59 -0800
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 21:00 -0800
                Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-18 08:25 -0500
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-18 18:24 -0800
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:59 -0800
                Re: std::atomic<T>::is_lock_free() Ian Collins <ian-news@hotmail.com> - 2019-11-16 11:08 +1300
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-15 15:39 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 07:22 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 08:06 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 17:13 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 08:26 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 19:27 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 10:37 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 19:47 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 11:34 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 09:17 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 01:52 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 11:23 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 03:22 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 14:20 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 06:39 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 15:49 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 22:54 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 08:03 +0100
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 15:39 -0800
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-28 16:33 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-10 22:48 -0800
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2020-01-11 00:24 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-14 07:16 -0800
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2020-01-14 08:55 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-21 02:01 -0800
                Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-21 06:00 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 15:58 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 21:01 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 07:00 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 22:45 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 07:52 +0100
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 08:10 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 23:23 -0800
                Re: std::atomic<T>::is_lock_free() Ian Collins <ian-news@hotmail.com> - 2019-11-18 20:26 +1300
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 23:05 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 15:44 -0800
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-28 16:43 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-11 06:41 -0800
        Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:33 -0800
  Re: std::atomic<T>::is_lock_free() Melzzzzz <Melzzzzz@zzzzz.com> - 2019-11-14 19:08 +0000
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 20:12 +0100
      Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:35 -0800
  Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:32 -0800
    Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:37 -0800
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 09:32 +0100
      Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:06 -0800
  Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 02:29 -0800
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 11:45 +0100

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