Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!aioe.org!news.glorb.com!border3.nntp.dca.giganews.com!Xl.tags.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local2.nntp.dca.giganews.com!news.giganews.com.POSTED!not-for-mail
NNTP-Posting-Date: Tue, 26 Apr 2011 13:30:04 -0500
From: Pete Becker <pete@versatilecoding.com>
Organization: Roundhouse Consulting, Ltd.
Newsgroups: comp.lang.c++
Date: Tue, 26 Apr 2011 14:30:04 -0400
Message-ID: <2011042614300497308-pete@versatilecodingcom>
References: <78f3178b-efdc-4af5-8f84-7ff6fa995af7@e25g2000prf.googlegroups.com> <d6774102-147b-4e3d-a457-a6bfd4c98a13@f15g2000pro.googlegroups.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Subject: Re: Please disprove this Double-Checked Locking "fix"
User-Agent: Unison/2.1.4
Lines: 32
X-Usenet-Provider: http://www.giganews.com
X-Trace: sv3-mlRk3Go5+Fse6/feW7mnFEoAbn6Vp8yr/noC3MB1IPD24XLkz4ArggoeenbGtqHTn9IDvHzTEefQrJN!qpxY4gOM84Gp0PfNo5tqQpVkMo1P3g73ABq1hVZ0FtdFVqYYrC0/zQWfTrOzBKhl0UxCL1EU
X-Complaints-To: abuse@giganews.com
X-DMCA-Notifications: http://www.giganews.com/info/dmca.html
X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers
X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly
X-Postfilter: 1.3.40
X-Original-Bytes: 2381
Xref: x330-a1.tempe.blueboxinc.net comp.lang.c++:4380

On 2011-04-26 14:16:33 -0400, Joshua Maurice said:

> This is largely due to hardware "reorderings",
> but let me quote the paper for the relevant bit:
> 
> [quote]
> Nothing you do can alter the fundamental problem: you need to be able
> to specify a constraint on instruction ordering, and your language
> gives you no way to do it.
> [/quote]
> 
> To emphasize, it might be the compiler reordering it, it might be the
> hardware reordering it,

Let me underscore the problem, as the quotation above misses part of 
the issue. Even when the instructions are executed in exactly the order 
that you want them to be executed, different threads can see results in 
a different order from the order in which the thread doing the stores 
actually did them.

When data is shared between threads and at least one thread is writing 
that data, all accesses to that data must be synchronized.

C++0x says that the behavior of a program that does not synchronize 
such accesses is undefined.

-- 
  Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The 
Standard C++ Library Extensions: a Tutorial and Reference 
(www.petebecker.com/tr1book)