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Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE
Date 2017-03-03 18:30 +0100
Message-ID <tgZbH-4pi-11@gated-at.bofh.it> (permalink)
References (3 earlier) <tgDbb-5KR-11@gated-at.bofh.it> <tgGLN-8cb-35@gated-at.bofh.it> <tgGLN-8cb-33@gated-at.bofh.it> <tgVB7-1Rv-7@gated-at.bofh.it> <tgYIG-3UX-27@gated-at.bofh.it>
Organization linux.* mail to news gateway

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On Fri, Mar 03, 2017 at 03:49:38PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 03, 2017 at 09:26:50AM +0100, Christian Borntraeger wrote:
> > Right. The main purpose is to read/write _ONCE_. You can assume a somewhat
> > atomic access for sizes <= word size. And there are certainly places that
> > rely on that. But the *ONCE thing is mostly used for things where we used
> > barrier() 10 years ago.
> 
> A lot of code relies on READ/WRITE_ONCE() to generate single
> instructions for naturally aligned machined word sized loads/stores
> (something GCC used to guarantee, but does no longer IIRC).
> 
> So much so that I would say its a bug if READ/WRITE_ONCE() doesn't
> generate a single instruction under those conditions.
> 
> However, every time I've tried to introduce stricter
> semantics/primitives to verify things Linus hated it.

See here for the last attempt:

  https://marc.info/?l=linux-virtualization&m=148007765918101&w=2

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[PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Arnd Bergmann <arnd@arndb.de> - 2017-03-02 18:00 +0100
  Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Christian Borntraeger <borntraeger@de.ibm.com> - 2017-03-02 18:00 +0100
    Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Arnd Bergmann <arnd@arndb.de> - 2017-03-02 19:00 +0100
      Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Arnd Bergmann <arnd@arndb.de> - 2017-03-02 22:50 +0100
        Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Arnd Bergmann <arnd@arndb.de> - 2017-03-03 14:40 +0100
        Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Christian Borntraeger <borntraeger@de.ibm.com> - 2017-03-03 14:40 +0100
          Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Peter Zijlstra <peterz@infradead.org> - 2017-03-03 18:00 +0100
            Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Peter Zijlstra <peterz@infradead.org> - 2017-03-03 18:30 +0100
      Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE Christian Borntraeger <borntraeger@de.ibm.com> - 2017-03-03 04:20 +0100

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