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<title>Issue 2034: Initialization of atomics is misspecified so that it doesn't preserve sequential consistency</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#Resolved">Resolved</a> status.</em></p>
<h3 id="2034"><a href="lwg-defects.html#2034">2034</a>. Initialization of atomics is misspecified so that it doesn't preserve sequential consistency</h3>
<p><b>Section:</b> 32.5.4 <a href="https://wg21.link/atomics.order">[atomics.order]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Hans Boehm <b>Opened:</b> 2011-02-26 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#atomics.order">active issues</a> in [atomics.order].</p>
<p><b>View all other</b> <a href="lwg-index.html#atomics.order">issues</a> in [atomics.order].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This violates the core intent of the memory model, as stated in the note in 6.10.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a> p. 21.
</p>

<p>
This was discovered by Mark Batty, and pointed out in their POPL 2011 paper, "Mathematizing C++ Concurrency",
section 4, "Sequential consistency of SC atomics".  The problem is quite technical, but well-explained in that paper.
</p>

<p>
This particular issue was not understood at the time the FCD comments were generated. But it is closely related 
to a number of FCD comments. It should have arisen from 
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3224.html#US171">US-171</a>, though that's not 
the actual history.
</p>

<p>This issue has been under discussion for several months in a group that included a half dozen or so of the 
most interested committee members. The P/R represents a well-considered consensus among us:</p>

<p><i>[2011-03-16: Jens updates wording]</i></p>


<blockquote class="note">
<p>Modify 32.5.4 <a href="https://wg21.link/atomics.order">[atomics.order]</a> p.3, so that the normative part reads:</p>

<blockquote><p>
3 There shall be a single total order <i>S</i> on all <code>memory_order_seq_cst</code> operations, consistent 
with the &quot;happens before&quot; order and modification orders for all affected locations, such that each 
<code>memory_order_seq_cst</code> operation that loads a value observes either the last 
preceding modification according to this order <i>S</i><ins>, <i>A</i> (if any)</ins>, or the result of an operation 
<ins><i>X</i></ins> that <del>is not</del><ins>does not happen before <i>A</i> and that is not</ins> <code>memory_order_seq_cst</code>. 
[ <i>Note</i>: Although it is not explicitly required that <i>S</i> include locks, it can always be extended 
to an order that does include lock and unlock operations, since the ordering between those is already included 
in the &quot;happens before&quot; ordering. &mdash; <i>end note</i> ]
</p></blockquote>

</blockquote>


<p id="res-2034"><b>Proposed resolution:</b></p><p>
Resolved 2011-03 Madrid meeting by paper <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3278">N3278</a>
</p>




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