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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-42370] Race between SharedStateImpl::addChild and transitionToFinished</title>
                <link>https://jira.mongodb.org/browse/SERVER-42370</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;There&apos;s an unfortunate race between addChild and transitionToFinished, where the caller of addChild can observe a kInit shared state, as can transitionToFinished, and then the caller of addChild can add a child that is never fulfilled by the caller of transitionToFinished.&lt;/p&gt;

&lt;p&gt;It looks like:&lt;/p&gt;

&lt;div class=&apos;table-wrap&apos;&gt;
&lt;table class=&apos;confluenceTable&apos;&gt;&lt;tbody&gt;
&lt;tr&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;addChild()&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;transitionToFinished()&lt;/th&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; loadState, observe init&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;&amp;nbsp;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; acquire mutex &lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;&amp;nbsp;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; loadState, observe init &lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;&amp;nbsp;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;&amp;nbsp;&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; transition to finished, observed init &lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; attempt to cas to waitingOrHaveChildren &lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;&amp;nbsp;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;&amp;nbsp;&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; return, doing nothing &lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; add to children &lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;&amp;nbsp;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; strand the child in a way that won&apos;t be finished later &lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;&amp;nbsp;&lt;/td&gt;
&lt;/tr&gt;
&lt;/tbody&gt;&lt;/table&gt;
&lt;/div&gt;
</description>
                <environment></environment>
        <key id="872540">SERVER-42370</key>
            <summary>Race between SharedStateImpl::addChild and transitionToFinished</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="mira.carey@mongodb.com">Mira Carey</assignee>
                                    <reporter username="mira.carey@mongodb.com">Mira Carey</reporter>
                        <labels>
                    </labels>
                <created>Tue, 23 Jul 2019 23:07:21 +0000</created>
                <updated>Sun, 29 Oct 2023 22:18:50 +0000</updated>
                            <resolved>Wed, 24 Jul 2019 17:34:29 +0000</resolved>
                                                    <fixVersion>4.2.0-rc5</fixVersion>
                    <fixVersion>4.3.1</fixVersion>
                                    <component>Internal Code</component>
                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="2342185" author="xgen-internal-githook" created="Wed, 24 Jul 2019 19:01:47 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Jason Carey&apos;, &apos;email&apos;: &apos;jcarey@argv.me&apos;, &apos;username&apos;: &apos;hanumantmk&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-42370&quot; title=&quot;Race between SharedStateImpl::addChild and transitionToFinished&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-42370&quot;&gt;&lt;del&gt;SERVER-42370&lt;/del&gt;&lt;/a&gt; Fix SharedStateImpl::addChild()&lt;/p&gt;

&lt;p&gt;There&apos;s an unfortunate race between addChild and transitionToFinished,&lt;br/&gt;
where the caller of addChild can observe a kInit shared state, as can&lt;br/&gt;
transitionToFinished, and then the caller of addChild can add a child&lt;br/&gt;
that is never fulfilled by the caller of transitionToFinished.&lt;/p&gt;

&lt;p&gt;It looks like:&lt;/p&gt;

&lt;p&gt;addChild()              | transitionToFinished()&lt;br/&gt;
==============================================&lt;br/&gt;
loadState, see init     |&lt;br/&gt;
acquire mutex           |&lt;br/&gt;
loadState, see init     |&lt;/p&gt;
&lt;div class=&apos;table-wrap&apos;&gt;
&lt;table class=&apos;confluenceTable&apos;&gt;&lt;tbody&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; transition to finished, observed init&lt;br/&gt;
attempt to cas, fail    &lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; return, doing nothing&lt;br/&gt;
add to children         &lt;/td&gt;
&lt;/tr&gt;
&lt;/tbody&gt;&lt;/table&gt;
&lt;/div&gt;

&lt;p&gt;strand the child        |&lt;/p&gt;

&lt;p&gt;The fix is to check to see if our compare_exchange failed, in which case&lt;br/&gt;
we need to fulfill ourselves&lt;/p&gt;

&lt;p&gt;(cherry picked from commit e82c7b495f4c4cebf12f4f1a19c2922b1d1d2683)&lt;br/&gt;
Branch: v4.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/aa5fef386e734d42ad8df1d0d37021daf38fc56c&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/aa5fef386e734d42ad8df1d0d37021daf38fc56c&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2341976" author="xgen-internal-githook" created="Wed, 24 Jul 2019 17:30:42 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Jason Carey&apos;, &apos;email&apos;: &apos;jcarey@argv.me&apos;, &apos;username&apos;: &apos;hanumantmk&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-42370&quot; title=&quot;Race between SharedStateImpl::addChild and transitionToFinished&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-42370&quot;&gt;&lt;del&gt;SERVER-42370&lt;/del&gt;&lt;/a&gt; Fix SharedStateImpl::addChild()&lt;/p&gt;

&lt;p&gt;There&apos;s an unfortunate race between addChild and transitionToFinished,&lt;br/&gt;
where the caller of addChild can observe a kInit shared state, as can&lt;br/&gt;
transitionToFinished, and then the caller of addChild can add a child&lt;br/&gt;
that is never fulfilled by the caller of transitionToFinished.&lt;/p&gt;

&lt;p&gt;It looks like:&lt;/p&gt;

&lt;p&gt;addChild()              | transitionToFinished()&lt;br/&gt;
==============================================&lt;br/&gt;
loadState, see init     |&lt;br/&gt;
acquire mutex           |&lt;br/&gt;
loadState, see init     |&lt;/p&gt;
&lt;div class=&apos;table-wrap&apos;&gt;
&lt;table class=&apos;confluenceTable&apos;&gt;&lt;tbody&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; transition to finished, observed init&lt;br/&gt;
attempt to cas, fail    &lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt; return, doing nothing&lt;br/&gt;
add to children         &lt;/td&gt;
&lt;/tr&gt;
&lt;/tbody&gt;&lt;/table&gt;
&lt;/div&gt;

&lt;p&gt;strand the child        |&lt;/p&gt;

&lt;p&gt;The fix is to check to see if our compare_exchange failed, in which case&lt;br/&gt;
we need to fulfill ourselves&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/e82c7b495f4c4cebf12f4f1a19c2922b1d1d2683&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/e82c7b495f4c4cebf12f4f1a19c2922b1d1d2683&lt;/a&gt;&lt;/p&gt;</comment>
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