Set status=SG1 and priority for 4174

Author: jwakely

xml/issue4174.xml

@@ -1,12 +1,12 @@
 <?xml version='1.0' encoding='utf-8' standalone='no'?>
 <!DOCTYPE issue SYSTEM "lwg-issue.dtd">
 
-<issue num="4174" status="New">
+<issue num="4174" status="SG1">
 <title>How does [atomics.order] p3 apply when then modification is an initialization?</title>
 <section><sref ref="[atomics.order]"/></section>
 <submitter>jim x</submitter>
 <date>13 Nov 2024</date>
-<priority>99</priority>
+<priority>3</priority>
 
 <discussion>
 <p>
@@ -46,6 +46,65 @@ According to <sref ref="[atomics.types.operations]"/> p3 (emphasize mine)
 So, how does <sref ref="[atomics.order]"/> p3 apply to this example such that the load operation precedes 
 the store operation in the single total order <i>S</i>?
 </p>
+
+<note>2025-02-07; Reflector poll</note>
+<p>
+Set priority to 3 after reflector poll. Send to SG1.
+</p>
+<p>
+LWG found the issue unclear and felt it was missing context that would
+help understand it properly.
+</p>
+<p>
+In
+<a href="https://github.com/cplusplus/CWG/issues/641">cplusplus/CWG/issues/641</a>
+the following example was given:
+<blockquote>
+<pre>
+std::atomic&lt;bool&gt; a = false;
+std::atomic&lt;bool&gt; b = false;
+int v = 0;
+// thread 1:
+a.store(true, seq_cst);
+if(b.load(seq_cst)== false){
+   v = 1;  // #1
+}
+//thread 2:
+b.store(true, seq_cst);
+if(a.load(seq_cst)== false){
+   v = 2; // #2
+}
+</pre>
+To prove whether #1 and #2 can have data race, we should prove whether
+it's possible that `a` and `b` simultaneously read `false`.
+This proof equals whether there can be a valid single total order in this case.
+To determine the order of `b.load` and `b.store` when `b.load` reads
+the initialization value `false`, <sref ref="[atomics.order]"/> p3.3
+should apply here.
+However, the initialization is not an atomic modification
+such that `X` cannot be that value.
+</blockquote>
+</p>
+
+<p>
+A possible fix is to amend <sref ref="[atomics.order]"/>/3.3 to say something
+like this:
+<blockquote>
+(3.3) A and B are not the same atomic read-modify-write operation, and
+<ins>either</ins>
+<ol style="list-style-type: none">
+<li><ins>(3.3.1)</ins>
+there exists an atomic modification X of M such that A reads
+the value stored by X and X precedes B in the modification order of M<ins>,
+or</ins>
+</li>
+<li><ins>(3.3.2)
+A reads the initial value of X and B modifies M</ins>, or
+</li>
+</ol>
+</blockquote>
+</p>
+
 </discussion>
 
 <resolution>