marsqing created HTTPCORE-746:
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Summary: Deadlock with Java Virtual Threads
Key: HTTPCORE-746
URL: https://issues.apache.org/jira/browse/HTTPCORE-746
Project: HttpComponents HttpCore
Issue Type: Improvement
Components: HttpCore
Affects Versions: 4.4.16
Environment: openjdk version "20.0.1" 2023-04-18
archlinux
Reporter: marsqing
Attachments: LoomWithApacheHttpClientTest.java
[Virtual Threads|https://openjdk.org/jeps/444] is expected to be released in
JDK21 soon. Contrast to traditional 'platform threads', virtual threads will
usually unmount from platform threads when doing blocking operations. But
accoring to the doc above, when doing blocking operations in synchronized
block/method, the carrying platform thread will be pinned and can not carry
other virtual threads.
This limitation will make the sample in the attachment deadlocked. Because all
the platform threads add pinned in
[await|https://github.com/apache/httpcomponents-core/blob/45d8a4cb391f591bb8d380b820437b651435774f/httpcore/src/main/java/org/apache/http/pool/AbstractConnPool.java#LL391C30-L391C30]
because it is in a [synchronized
block|https://github.com/apache/httpcomponents-core/blob/45d8a4cb391f591bb8d380b820437b651435774f/httpcore/src/main/java/org/apache/http/pool/AbstractConnPool.java#L244]
and no connection is available in the pool. The only virtual thread which can
release the connection to the pool is waiting for some platform threads to
carry it. That is the platform thread is waiting for a connection while the
connection is waiting the platform thread to carry its virtual thread and
return the connection. Thus a deadlock.
If we replace the synchronized above with a ReentrantLock, no thead will be
pinned and no deadlock will occur. Accoring to the doc above, the limitation
may be removed in future JDK versions. But before that, we may need to replace
synchronized with some locks. Do we have any related plan? Thanks!
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