New issue from jim x: "The load and store operation in §[atomics.orde…

…r] p1 is ambiguous"

xml/issue4004.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4004" status="New">
+<title>The load and store operation in &sect;[atomics.order] p1 is ambiguous</title>
+<section><sref ref="[atomics.order]"/></section>
+<submitter>jim x</submitter>
+<date>30 Oct 2023</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+<sref ref="[atomics.order]"/> p1 says:
+</p>
+<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
+<ol style="list-style-type: none">
+<li><p>(1.2) &mdash; <tt>memory_order::release</tt>, <tt>memory_order::acq_rel</tt>, and <tt>memory_order::seq_cst</tt>: a store operation 
+performs a release operation on the affected memory location.</p></li>
+<li><p>(1.3) &mdash; <tt>memory_order::consume</tt>: a load operation performs a consume operation on the affected memory location. 
+[&hellip;]</p></li>
+<li><p>(1.4) &mdash; <tt>memory_order::acquire</tt>, <tt>memory_order::acq_rel</tt>, and <tt>memory_order::seq_cst</tt>: a 
+load operation performs an acquire operation on the affected memory location.</p></li>
+</ol>
+</blockquote>
+<p>
+What do the store and load operations intend to mean in this context? If there is no extra specification, 
+it is easy to consider them as the operations performed by the non-static member functions "store" and "load" 
+defined in the <tt>atomic</tt> class (template).
+<p/>
+<sref ref="[atomics.order]"/> p2 says
+</p>
+<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
+<p>
+An atomic operation <i>A</i> that performs a release operation on an atomic object <i>M</i> synchronizes with 
+an atomic operation <i>B</i> that performs an acquire operation on <i>M</i> and takes its value from any side 
+effect in the release sequence headed by <i>A</i>.
+</p>
+</blockquote>
+<p>
+According to the above interpretation, <i>A</i> is an operation performed by the non-static member function 
+<tt>store</tt>, however, I think the following example can establish the synchronization relationship
+</p>
+<blockquote><pre>
+std::atomic&lt;int&gt; x{0};
+Thread 1:
+int expected = 0;
+x.compare_exchange_strong(expected, 1, memory_order::release, memory_order::relaxed ); //#1
+
+Thread 2:
+int expected = 1;
+while(x.compare_exchange_strong( expected, 2, memory_order::acquire, memory_order::relaxed )){} // #2
+</pre></blockquote>
+<p>
+Assuming the RMW operations are successful in the two threads, I think <tt>#1</tt> intends to perform a 
+release operation while <tt>#2</tt> performs an acquire operation, and hence they can establish the 
+synchronization relationship, however, they both are RMW operations.
+<p/>
+It should be clearly defined which are store operations and which are load operations. 
+</p>
+</discussion>
+
+<resolution>
+</resolution>
+
+</issue>