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<title>Issue 3263: Atomic waiting function calls should only be unblocked once</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#New">New</a> status.</em></p>
<h3 id="3263"><a href="lwg-active.html#3263">3263</a>. Atomic waiting function calls should only be unblocked once</h3>
<p><b>Section:</b> 32.5.6 <a href="https://wg21.link/atomics.wait">[atomics.wait]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Geoffrey Romer <b>Opened:</b> 2019-08-19 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#atomics.wait">active issues</a> in [atomics.wait].</p>
<p><b>View all other</b> <a href="lwg-index.html#atomics.wait">issues</a> in [atomics.wait].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It appears that in a conforming implementation, all but one <code>wait()</code> call on a given 
atomic object may block forever, regardless of any <code>notify_one()</code> calls, because in 
principle every <code>notify_one()</code> call could be considered to unblock the same single 
<code>wait()</code> call. Common sense suggests (and David Olsen confirms) that the intent is 
for each waiting function call to be (non-spuriously) unblocked by at most one notifying 
function call, but as far as I can tell the words never say that.
</p>

<p><i>[2019-09-14 Priority set to 3 based on reflector discussion]</i></p>



<p id="res-3263"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 32.5.6 <a href="https://wg21.link/atomics.wait">[atomics.wait]</a> as indicated:</p>

<blockquote>
<p>
<ins>-?- All blocking and unblocking events on a single atomic object occur in a single total order 
that is consistent with the "happens before" partial order.</ins>
<p/>
-4- A call to an atomic waiting operation on an atomic object <code>M</code> is <i>eligible to be 
unblocked</i> by a call to an atomic notifying operation on <code>M</code> if <ins>it has not been 
unblocked, and</ins> there exist side effects <code>X</code> and <code>Y</code> on <code>M</code> such that:
</p>
<ol style="list-style-type: none">
<li><p>(4.1) &mdash; the atomic waiting operation has blocked after observing the result of <code>X</code>,</p></li>
<li><p>(4.2) &mdash; <code>X</code> precedes <code>Y</code> in the modification order of <code>M</code>, and</p></li>
<li><p>(4.3) &mdash; <code>Y</code> happens before the call to the atomic notifying operation.</p></li>
</ol>
</blockquote>
</li>

</ol>



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