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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> kernelnewbies-bounces@kernelnewbies.org [mailto:kernelnewbies-bounces@kernelnewbies.org]
<b>On Behalf Of </b>Malte Vesper<br>
<b>Sent:</b> Thursday, March 26, 2015 10:50 AM<br>
<b>To:</b> kernelnewbies<br>
<b>Subject:</b> Re: Atomics and memory barriers<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom:12.0pt">On a second thought:<br>
<br>
Is it correct that atomic_set() without a writebarrier following it is still atomic (no torn writes), however not guaranteed to be visible to other threads?<o:p></o:p></p>
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<p class="MsoNormal">On 26/03/15 17:47, Malte Vesper wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hello,<br>
I have been reading up on atomics and struggle to grasp when exactly I need explicit memory barriers.<br>
<br>
While the documentation (Documentat/atomic_ops.txt), talks about operations needing explicit barriers both sides, I assume this is only half true.<br>
If I for instance only want to use an atomic as a flag to show that some operation is complete I reckon that a barrier before is sufficient.<br>
<br>
Furthermore I found the following lines:<o:p></o:p></p>
<pre><span class="lno"><a href="http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L288" id="L288">288</a> </span><span class="code">If a caller requires memory barrier semantics around an atomic_t</span><o:p></o:p></pre>
<pre><span class="lno"><a href="http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L289" id="L289">289</a> </span><span class="code">operation which does not return a value, a set of interfaces are</span><o:p></o:p></pre>
<pre><span class="lno"><a href="http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L290" id="L290">290</a> </span><span class="code">defined which accomplish this:</span><o:p></o:p></pre>
<pre><span class="lno"><a href="http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L291" id="L291">291</a> </span><o:p></o:p></pre>
<pre><span class="lno"><a href="http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L292" id="L292">292</a> </span><span class="code"> void smp_mb__before_atomic(void);</span><o:p></o:p></pre>
<pre><span class="lno"><a href="http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L293" id="L293">293</a> </span><span class="code"> void smp_mb__after_atomic(void);</span><o:p></o:p></pre>
<p class="MsoNormal">note how it refers to "operation which does not return a value", why can't I use these for atomic operations that do return a value?<br>
What should I use instead, normal barriers like mb?<br>
<br>
Please enlighten me<br>
Malte<o:p></o:p></p>
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<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">You might want to try reading Documentation/memory-barriers.txt.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">It does a better job of explaining this stuff than atomic_ops.txt.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">That being said, none of this stuff is easy to understand.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I’ve read both a few times and still don’t quite get all of it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Good luck,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Jeff Haran<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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