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Hi George,<br>
<br>
I just noticed that my reply never made it to the list.<br>
<br>
Thanks for raising awareness of this issue!</div>
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Cheers,<br>
–</div>
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<br>
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<b><br>
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<b>Viktor Klang</b></div>
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Software Architect, Java Platform Group<br>
Oracle</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> concurrency-discuss <concurrency-discuss-retn@openjdk.org> on behalf of George Lu <george.lu@broadcom.com><br>
<b>Sent:</b> Friday, 28 February 2025 21:51<br>
<b>To:</b> concurrency-discuss@openjdk.org <concurrency-discuss@openjdk.org><br>
<b>Subject:</b> ForkJoinPool TC underflow issue in JDK 11 and 17 (JDK-8330017)</font>
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<div class="PlainText">Hi all,<br>
<br>
There is a known issue (JDK-8330017) in JDK 11 and 17 where the<br>
ForkJoinPool can hang when the Release Count (RC) reaches<br>
ForkJoinPool.MAX_CAP.<br>
<br>
This occurs due to an underflow in Total Count (TC) when attempting to<br>
signal for deregisterWorker. Specifically, in JDK 17, the following<br>
line:<br>
<br>
compareAndSetCtl(c, ((UC_MASK & (c - TC_UNIT)) | (prevCtl & SP_MASK))))<br>
<br>
When TC == 0, (c - TC_UNIT) underflows, unintentionally decrementing<br>
RC by 1. Notably, TC == 0 is not a rare edge case (since total workers<br>
== parallelism is common), meaning this accidental underflow can occur<br>
frequently. We have encountered this issue in two of our long-running<br>
JVMs. A possible fix is to properly mask TC and RC as follows:<br>
<br>
compareAndSetCtl(c, ((RC_MASK & c) | (TC_MASK & (c - TC_UNIT)) |<br>
(prevCtl & SP_MASK))))<br>
<br>
Regards,<br>
George<br>
<br>
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