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This is an automatically generated e-mail. To reply, visit:
<a href="https://git.reviewboard.kde.org/r/121134/">https://git.reviewboard.kde.org/r/121134/</a>
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<p style="margin-top: 0;">On November 16th, 2014, 7:22 p.m. UTC, <b>Thomas Lübking</b> wrote:</p>
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<a href="https://git.reviewboard.kde.org/r/121134/diff/5/?file=328836#file328836line343" style="color: black; font-weight: bold; text-decoration: underline;">kdecore/kernel/kglobal.cpp</a>
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(Diff revision 5)
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<td colspan="4"><pre style="font-size: 8pt; line-height: 140%; margin: 0; ">QString KGlobal::caption()</pre></td>
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<td bgcolor="#fdfebc" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "> <span class="n">s_allowQuit</span><span class="hl"> </span><span class="o"><span class="hl">=</span></span><span class="hl"> </span><span class="n">allowQuit</span><span class="p">;</span></pre></td>
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<td bgcolor="#fdfebc" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "> <span class="n">s_allowQuit</span><span class="p"><span class="hl">.</span></span><span class="n"><span class="hl">fetchAndStoreOrdered</span></span><span class="p"><span class="hl">(</span></span><span class="kt"><span class="hl">int</span></span><span class="p"><span class="hl">(</span></span><span class="n">allowQuit</span><span class="p"><span class="hl">))</span>;</span></pre></td>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">Does this actually make sense?
The result is "undefined" when used from multiple threads anyway.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">Should one enforce this to be called from the main thread only instead?</p></pre>
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<p>On November 16th, 2014, 8:08 p.m. UTC, <b>René J.V. Bertin</b> wrote:</p>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">How do you mean, undefined? The result will be the value set by whoever called the function last.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">You have a point that this doesn't make a lot of sense, there's something to say that the main (master...) thread ought to be the one deciding whether or not it can be quit by the reference counter.</p></pre>
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<p>On November 16th, 2014, 8:38 p.m. UTC, <b>Thomas Lübking</b> wrote:</p>
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<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">How do you mean, undefined?</p>
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<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">It's a write-only flag. If written from two threads with disagree, none has an idea about the current state - and there's no getter either.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">Since the client code somehow has to find agreement on the outcome, it needs a local mutex - or define who's in position to write this ("the one deciding whether or not it can be quit by the reference counter")</p></pre>
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<pre style="margin-left: 1em; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">A local mutex around that one line would be completely pointless. The effect will be exactly the same: no data race (atomic operations don't participate in data races), but what I call a "semantic" race, since the last writer wins.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">However, let's not invent problems. setAllowQuit is always called by the main thread, and only with true as argument (i.e. this is about moving from the initial "don't quit yet it's too early" to the "ok, normal operations start here").
The only reason for an atomic int here is so that the reading by secondary threads doesn't lead to a data race (with the write by the main thread).</p></pre>
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<p>- David</p>
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<p>On November 16th, 2014, 8:22 p.m. UTC, René J.V. Bertin wrote:</p>
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<div>Review request for KDE Software on Mac OS X, kdelibs and David Faure.</div>
<div>By René J.V. Bertin.</div>
<p style="color: grey;"><i>Updated Nov. 16, 2014, 8:22 p.m.</i></p>
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<b style="color: #575012; font-size: 10pt;">Repository: </b>
kdelibs
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Description </h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">I have been experiencing unexpected exits from KDevelop that were not due to any kind of error in the KDevelop code; it was as if someone told the application to exit normally. This happens mostly on OS X, but also sometimes on Linux.
I finally traced this to <code style="text-rendering: inherit;color: #4444cc;padding: 0;white-space: normal;margin: 0;line-height: inherit;">KGlobal::deref</code> reaching a zero reference count and invoking <code style="text-rendering: inherit;color: #4444cc;padding: 0;white-space: normal;margin: 0;line-height: inherit;">QCoreApplication::quit</code> when called from one of KDevelop's KJob dtors. There does not appear to be a reference counting mismatch in the code, so the issue might be due to a race condition in KGlobal::ref/deref.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">This patch introduces thread-safety to KGlobal's reference counting by turning the simple global <code style="text-rendering: inherit;color: #4444cc;padding: 0;white-space: normal;margin: 0;line-height: inherit;">static int s_refCount</code> into a <code style="text-rendering: inherit;color: #4444cc;padding: 0;white-space: normal;margin: 0;line-height: inherit;">static QAtomicInt s_refCount</code>. I consider this an important bug fix regardless of whether it corrects the issue I have with KDevelop.</p></pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Testing </h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">On OS X 10.6.8 only for now.</p>
<p style="padding: 0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">NB: Qt itself uses the QAtomicInt's parent class (QBasicAtomicInt) for its own reference counter used for implicit sharing. This is thus well-tested code that is unlikely to introduce regressions to a core KDE feature.</p></pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
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<li>kdecore/kernel/kglobal.cpp <span style="color: grey">(cf003a4)</span></li>
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<p><a href="https://git.reviewboard.kde.org/r/121134/diff/" style="margin-left: 3em;">View Diff</a></p>
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