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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-55949] Relaxed load in CancellationState::isCanceled can allow for inconsistent state to be read</title>
                <link>https://jira.mongodb.org/browse/SERVER-55949</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Consider a cancelation hierarchy as follows:&lt;/p&gt;
&lt;p/&gt;
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		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;CancelationSource first;&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;CancelationSource second(first.token());&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;Now imagine some thread, thread A, calls&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;first.cancel();&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;Around the same time, another thread B reads the cancellation states:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;if&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; (second.isCanceled()) &lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;     &lt;/span&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;if&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt; (first.isCancelled())     &lt;/span&gt;&lt;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;else&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;...&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;Based on my understanding of &lt;a href=&quot;https://en.cppreference.com/w/cpp/atomic/memory_order&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;memory ordering&lt;/a&gt;, the following could happen:&lt;/p&gt;

&lt;p&gt;When thread A cancels the first source, it marks the cancelation state for that source as &quot;canceled&quot; and then emplaces its cancelation promise. Because the second source is in a hierarchy with the first source, fulfilling this promise causes &lt;a href=&quot;https://github.com/mongodb/mongo/blob/af4b105ce8cbbcd5ece444c5167211fd7321f379/src/mongo/util/cancellation.h#L239&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;this&lt;/a&gt; continuation to run inline on thread A, which marks the cancelation state for the second source as &quot;canceled.&quot;&#160;&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;However, because CancelationState::isCanceled uses loadRelaxed, it is possible that thread B reads second.isCanceled() == true and first.isCanceled == false; even though the stores are ordered first --&amp;gt; second on thread A, the relaxed load on thread B won&apos;t force any synchronization with the system. It is permissible in the memory model for thread B to read second.isCanceled without being synchronized with all the things that happened before the second source was cancelled in thread A. (The best explanation of this for me was the last example, &quot;Mixing Memory Models&quot; on this page: &lt;a href=&quot;https://gcc.gnu.org/wiki/Atomic/GCCMM/AtomicSync)&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://gcc.gnu.org/wiki/Atomic/GCCMM/AtomicSync)&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;Tenant migrations and resharding code wants to rely on the ordering of cancelation sources in a hierarchy to be consistently visible to other threads; I think this goal makes sense and we should forbid reading this inconsistent state.&lt;/p&gt;

&lt;p&gt;Thanks to &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=max.hirschhorn&quot; class=&quot;user-hover&quot; rel=&quot;max.hirschhorn&quot;&gt;max.hirschhorn&lt;/a&gt;&#160;for finding this and working through the memory model examples with me.&#160;&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;&lt;b&gt;Acceptance Criteria:&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;isCanceled() uses load instead of&#160;loadRelaxed.&#160;&lt;/p&gt;</description>
                <environment></environment>
        <key id="1675237">SERVER-55949</key>
            <summary>Relaxed load in CancellationState::isCanceled can allow for inconsistent state to be read</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="alex.li@mongodb.com">Alex Li</assignee>
                                    <reporter username="george.wangensteen@mongodb.com">George Wangensteen</reporter>
                        <labels>
                            <label>post-rc0</label>
                            <label>servicearch-wfbf-day</label>
                    </labels>
                <created>Thu, 8 Apr 2021 21:15:21 +0000</created>
                <updated>Sun, 29 Oct 2023 21:55:07 +0000</updated>
                            <resolved>Thu, 17 Jun 2021 20:47:48 +0000</resolved>
                                                    <fixVersion>5.0.0-rc4</fixVersion>
                    <fixVersion>5.1.0-rc0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="4108002" author="JIRAUSER1259052" created="Wed, 6 Oct 2021 18:54:01 +0000"  >&lt;p&gt;Updating the fixversion since branching activities occurred yesterday. This ticket will be in rc0 when it&#8217;s been triggered. For more active release information, please keep an eye on #server-release. Thank you!&lt;/p&gt;</comment>
                            <comment id="3894392" author="xgen-internal-githook" created="Wed, 23 Jun 2021 16:15:59 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Alex Li&apos;, &apos;email&apos;: &apos;alex.li@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-55949&quot; title=&quot;Relaxed load in CancellationState::isCanceled can allow for inconsistent state to be read&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-55949&quot;&gt;&lt;del&gt;SERVER-55949&lt;/del&gt;&lt;/a&gt; CancellationState::isCanceled and isCancelable use load instead of loadRelaxed&lt;/p&gt;

&lt;p&gt;(cherry picked from commit 7916b72cd1082330e05d5fd052e099f3387ca513)&lt;br/&gt;
Branch: v5.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/1f68be5545592b9cb7ee8593422760bad714101e&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/1f68be5545592b9cb7ee8593422760bad714101e&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3886058" author="xgen-internal-githook" created="Thu, 17 Jun 2021 21:57:50 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Alex Li&apos;, &apos;email&apos;: &apos;alex.li@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-55949&quot; title=&quot;Relaxed load in CancellationState::isCanceled can allow for inconsistent state to be read&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-55949&quot;&gt;&lt;del&gt;SERVER-55949&lt;/del&gt;&lt;/a&gt; CancellationState::isCanceled and isCancelable use load instead of loadRelaxed&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/7916b72cd1082330e05d5fd052e099f3387ca513&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/7916b72cd1082330e05d5fd052e099f3387ca513&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3784757" author="billy.donahue" created="Tue, 18 May 2021 16:42:08 +0000"  >&lt;p&gt;Would this problem also apply to the &lt;tt&gt;loadRelaxed()&lt;/tt&gt; call in &lt;tt&gt;isCancelable()&lt;/tt&gt; ?&lt;/p&gt;</comment>
                            <comment id="3777383" author="JIRAUSER1253452" created="Mon, 17 May 2021 16:35:05 +0000"  >&lt;p&gt;CR -&#160;&lt;a href=&quot;https://mongodbcr.appspot.com/788920003/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://mongodbcr.appspot.com/788920003/&lt;/a&gt;&lt;/p&gt;</comment>
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