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Re: Please disprove this Double-Checked Locking "fix"

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Joshua Maurice wrote:

> On May 19, 9:46 am, Gerhard Fiedler <geli...@gmail.com> wrote:
>> To go back to the example, I need to have the guarantee that blah2
>> runs unlocked (within the overall scheduling granularity of the
>> target system, of course).
> 
> As I said earlier, good luck attempting to write a portable
> specification to that end, and you don't really "need" it. You need
> it as much as you need more than 10 bytes of stack space, and you
> need it as much as you need a compiler that won't maliciously
> pessimize your programs. If you need more than that, you need
> something more than what a portable standard can give you, and you
> need to check / write the assembly yourself or maybe use some
> realtime POSIX extensions.

Check out this document
<http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.85.9908> about
the pthread guarantees. I haven't yet read (and understood) all of it,
but it seems to me that in proves that both lock and unlock require
memory fences (albeit different ones), and that it is not allowed to
reorder memory operations across locks (and only in a specific direction
across unlocks). 

I interpret this such that a compiler (that claims to implement the
pthread spec) is not allowed to remove a lock operation -- even if
there's nothing between the lock and the following unlock, and even if
it is to combine two lock/unlock pairs --, as this would remove a memory
fence and allow operations to be reordered across a code position across
which they weren't allowed to be reordered before removing the lock
operation.

Gerhard

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Thread

Please disprove this Double-Checked Locking "fix" jl_post@hotmail.com - 2011-04-26 09:58 -0700
  Re: Please disprove this Double-Checked Locking "fix" Leigh Johnston <leigh@i42.co.uk> - 2011-04-26 18:17 +0100
  Re: Please disprove this Double-Checked Locking "fix" Pete Becker <pete@versatilecoding.com> - 2011-04-26 13:50 -0400
    Re: Please disprove this Double-Checked Locking "fix" Scott Meyers <NeverRead@aristeia.com> - 2011-05-01 17:14 -0700
      Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-05-02 15:59 -0700
        Re: Please disprove this Double-Checked Locking "fix" Pete Becker <pete@versatilecoding.com> - 2011-05-03 08:39 -0400
  Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-04-26 11:16 -0700
    Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-04-26 11:19 -0700
    Re: Please disprove this Double-Checked Locking "fix" Pete Becker <pete@versatilecoding.com> - 2011-04-26 14:30 -0400
      Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-04-26 11:50 -0700
    Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-05-11 19:55 -0700
      Re: Please disprove this Double-Checked Locking "fix" Gerhard Fiedler <gelists@gmail.com> - 2011-05-13 19:56 -0300
        Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-05-13 16:59 -0700
          Re: Please disprove this Double-Checked Locking "fix" Gerhard Fiedler <gelists@gmail.com> - 2011-05-18 18:12 -0300
            Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-05-18 14:53 -0700
              Re: Please disprove this Double-Checked Locking "fix" Gerhard Fiedler <gelists@gmail.com> - 2011-05-19 13:46 -0300
                Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-05-19 15:30 -0700
                Re: Please disprove this Double-Checked Locking "fix" Gerhard Fiedler <gelists@gmail.com> - 2011-05-21 11:55 -0300
                Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-05-22 01:08 -0700
  Re: Please disprove this Double-Checked Locking "fix" James Kanze <james.kanze@gmail.com> - 2011-04-30 15:54 -0700
    Re: Please disprove this Double-Checked Locking "fix" Leigh Johnston <leigh@i42.co.uk> - 2011-05-01 21:49 +0100
      Re: Please disprove this Double-Checked Locking "fix" Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2011-05-01 17:26 -0400
        Re: Please disprove this Double-Checked Locking "fix" Leigh Johnston <leigh@i42.co.uk> - 2011-05-01 22:44 +0100
          Re: Please disprove this Double-Checked Locking "fix" Leigh Johnston <leigh@i42.co.uk> - 2011-05-02 01:01 +0100
            Re: Please disprove this Double-Checked Locking "fix" Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2011-05-01 22:04 -0400
            Re: Please disprove this Double-Checked Locking "fix" "Chris M. Thomasson" <cristom@charter.net> - 2011-05-04 11:49 -0700
              Re: Please disprove this Double-Checked Locking "fix" Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2011-05-06 00:16 -0400
          Re: Please disprove this Double-Checked Locking "fix" Joshua Maurice <joshuamaurice@gmail.com> - 2011-05-02 15:43 -0700
        Re: Please disprove this Double-Checked Locking "fix" James Kanze <james.kanze@gmail.com> - 2011-05-01 14:53 -0700
          Re: Please disprove this Double-Checked Locking "fix" Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2011-05-01 19:23 -0400
            Re: Please disprove this Double-Checked Locking "fix" James Kanze <james.kanze@gmail.com> - 2011-05-02 09:02 -0700
              Re: Please disprove this Double-Checked Locking "fix" Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2011-05-05 23:46 -0400
      Re: Please disprove this Double-Checked Locking "fix" James Kanze <james.kanze@gmail.com> - 2011-05-01 14:47 -0700

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