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From "Lyor Goldstein (Jira)" <j...@apache.org>
Subject [jira] [Commented] (SSHD-966) Deadlock on disconnection at the end of key-exchange
Date Thu, 19 Mar 2020 14:58:00 GMT

    [ https://issues.apache.org/jira/browse/SSHD-966?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17062666#comment-17062666
] 

Lyor Goldstein commented on SSHD-966:
-------------------------------------

I have tried several workarounds - but none successful. This is a tough one - still looking....
 Basically, the problem is that we need to flush all the pending messages that were accumulated
while KEX was in progress and then mark KEX as DONE. However, we must do this while preventing
other packets from being sent until flush is complete. I have thought about using a dedicated
+thread+ that waits on a pending outgoing queue of messages - it would solve this issue, but
raise others - e.g., the need to return an {{IOWriteFuture}} from {{writePacket}} (possible,
but adds non-trivial complexity).

any ideas ?

> Deadlock on disconnection at the end of key-exchange
> ----------------------------------------------------
>
>                 Key: SSHD-966
>                 URL: https://issues.apache.org/jira/browse/SSHD-966
>             Project: MINA SSHD
>          Issue Type: Bug
>    Affects Versions: 2.0.0
>            Reporter: Francois Ferrand
>            Assignee: Lyor Goldstein
>            Priority: Major
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> We are using git-repo to download projects from Gerrit server, using SSH.
> Gerrit is in version 2.16.16. which uses SSHD 2.0.0 and Mina 2.0.17 with NIO2 backend.
> One particularity of this setup is that git-repo creates a single control master channel,
and then downloads *lots* of Git repositories (500 repositories, some of them relatively large),
with some degree of parallelism. This takes a long time, lots of data, and the multiplexed
connections are handled by gerrit in multiple threads.
> In some cases, we experience a deadlock when an error happens at the end of the key exchange,
while sending pending packets:
> {noformat}
> Warning, the following threads are deadlocked : SSH git-upload-pack /project1 (myuser),
sshd-SshServer[df5f657]-nio2-thread-3
> "SSH git-upload-pack /project1 (myuser)" prio=1 BLOCKED
> 	org.apache.sshd.common.session.helpers.AbstractSession.writePacket(AbstractSession.java:1107)
> 	org.apache.sshd.common.channel.AbstractChannel.writePacket(AbstractChannel.java:798)
> 	org.apache.sshd.common.channel.ChannelOutputStream.flush(ChannelOutputStream.java:227)
> 	org.apache.sshd.common.channel.ChannelOutputStream.write(ChannelOutputStream.java:127)
> 	org.eclipse.jgit.transport.UploadPack$ResponseBufferedOutputStream.write(UploadPack.java:2183)
> 	org.eclipse.jgit.transport.SideBandOutputStream.writeBuffer(SideBandOutputStream.java:174)
> 	org.eclipse.jgit.transport.SideBandOutputStream.write(SideBandOutputStream.java:153)
> 	org.eclipse.jgit.internal.storage.pack.PackOutputStream.write(PackOutputStream.java:132)
> 	org.eclipse.jgit.internal.storage.file.PackFile.copyAsIs2(PackFile.java:614)
> 	org.eclipse.jgit.internal.storage.file.PackFile.copyAsIs(PackFile.java:433)
> 	org.eclipse.jgit.internal.storage.file.WindowCursor.copyObjectAsIs(WindowCursor.java:221)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjectImpl(PackWriter.java:1644)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObject(PackWriter.java:1621)
> 	org.eclipse.jgit.internal.storage.pack.PackOutputStream.writeObject(PackOutputStream.java:171)
> 	org.eclipse.jgit.internal.storage.file.WindowCursor.writeObjects(WindowCursor.java:229)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjects(PackWriter.java:1609)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjects(PackWriter.java:1597)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writePack(PackWriter.java:1154)
> 	org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:2133)
> 	org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:1947)
> 	org.eclipse.jgit.transport.UploadPack.service(UploadPack.java:971)
> 	org.eclipse.jgit.transport.UploadPack.upload(UploadPack.java:776)
> 	com.google.gerrit.sshd.commands.Upload.runImpl(Upload.java:77)
> 	com.google.gerrit.sshd.AbstractGitCommand.service(AbstractGitCommand.java:98)
> 	com.google.gerrit.sshd.AbstractGitCommand.access$000(AbstractGitCommand.java:31)
> 	com.google.gerrit.sshd.AbstractGitCommand$1.run(AbstractGitCommand.java:63)
> 	com.google.gerrit.sshd.BaseCommand$TaskThunk.run(BaseCommand.java:467)
> 	com.google.gerrit.server.logging.LoggingContextAwareRunnable.run(LoggingContextAwareRunnable.java:83)
> 	java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
> 	java.util.concurrent.FutureTask.run(FutureTask.java:266)
> 	java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
> 	java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
> 	com.google.gerrit.server.git.WorkQueue$Task.run(WorkQueue.java:646)
> 	java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
> 	java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
> 	java.lang.Thread.run(Thread.java:748)
> "sshd-SshServer[df5f657]-nio2-thread-3" daemon prio=5 BLOCKED
> 	org.apache.sshd.common.channel.ChannelOutputStream.close(ChannelOutputStream.java:249)
> 	org.apache.sshd.common.util.io.IoUtils.closeQuietly(IoUtils.java:151)
> 	org.apache.sshd.server.channel.ChannelSession.doCloseImmediately(ChannelSession.java:205)
> 	org.apache.sshd.common.util.closeable.AbstractCloseable.close(AbstractCloseable.java:83)
> 	org.apache.sshd.common.channel.AbstractChannel.close(AbstractChannel.java:576)
> 	org.apache.sshd.common.util.closeable.ParallelCloseable.doClose(ParallelCloseable.java:65)
> 	org.apache.sshd.common.util.closeable.SimpleCloseable.close(SimpleCloseable.java:63)
> 	org.apache.sshd.common.util.closeable.AbstractInnerCloseable.doCloseImmediately(AbstractInnerCloseable.java:46)
> 	org.apache.sshd.common.util.closeable.AbstractCloseable.close(AbstractCloseable.java:83)
> 	org.apache.sshd.common.util.closeable.ParallelCloseable.doClose(ParallelCloseable.java:65)
> 	org.apache.sshd.common.util.closeable.SimpleCloseable.close(SimpleCloseable.java:63)
> 	org.apache.sshd.common.util.closeable.SequentialCloseable$1.operationComplete(SequentialCloseable.java:56)
> 	org.apache.sshd.common.util.closeable.SequentialCloseable$1.operationComplete(SequentialCloseable.java:45)
> 	org.apache.sshd.common.util.closeable.SequentialCloseable.doClose(SequentialCloseable.java:69)
> 	org.apache.sshd.common.util.closeable.SimpleCloseable.close(SimpleCloseable.java:63)
> 	org.apache.sshd.common.util.closeable.AbstractInnerCloseable.doCloseImmediately(AbstractInnerCloseable.java:46)
> 	org.apache.sshd.common.util.closeable.AbstractCloseable.close(AbstractCloseable.java:83)
> 	org.apache.sshd.common.session.helpers.AbstractSession.exceptionCaught(AbstractSession.java:973)
> 	org.apache.sshd.common.session.helpers.AbstractSessionIoHandler.exceptionCaught(AbstractSessionIoHandler.java:53)
> 	org.apache.sshd.common.io.nio2.Nio2Session.exceptionCaught(Nio2Session.java:186)
> 	org.apache.sshd.common.io.nio2.Nio2Session.handleWriteCycleFailure(Nio2Session.java:460)
> 	org.apache.sshd.common.io.nio2.Nio2Session$2.onFailed(Nio2Session.java:415)
> 	org.apache.sshd.common.io.nio2.Nio2CompletionHandler.lambda$failed$1(Nio2CompletionHandler.java:46)
> 	org.apache.sshd.common.io.nio2.Nio2CompletionHandler$$Lambda$627/1874891543.run(Unknown
Source)
> 	java.security.AccessController.doPrivileged(Native Method)
> 	org.apache.sshd.common.io.nio2.Nio2CompletionHandler.failed(Nio2CompletionHandler.java:45)
> 	sun.nio.ch.Invoker.invokeUnchecked(Invoker.java:128)
> 	sun.nio.ch.Invoker.invokeDirect(Invoker.java:157)
> 	sun.nio.ch.UnixAsynchronousSocketChannelImpl.implWrite(UnixAsynchronousSocketChannelImpl.java:736)
> 	sun.nio.ch.AsynchronousSocketChannelImpl.write(AsynchronousSocketChannelImpl.java:382)
> 	sun.nio.ch.AsynchronousSocketChannelImpl.write(AsynchronousSocketChannelImpl.java:399)
> 	org.apache.sshd.common.io.nio2.Nio2Session.doWriteCycle(Nio2Session.java:401)
> 	org.apache.sshd.common.io.nio2.Nio2Session.startWriting(Nio2Session.java:386)
> 	org.apache.sshd.common.io.nio2.Nio2Session.writePacket(Nio2Session.java:169)
> 	org.apache.sshd.common.session.helpers.AbstractSession.doWritePacket(AbstractSession.java:1181)
> 	org.apache.sshd.common.session.helpers.AbstractSession.sendPendingPackets(AbstractSession.java:910)
> 	org.apache.sshd.common.session.helpers.AbstractSession.handleNewKeys(AbstractSession.java:875)
> 	org.apache.sshd.common.session.helpers.AbstractSession.doHandleMessage(AbstractSession.java:606)
> 	org.apache.sshd.common.session.helpers.AbstractSession.handleMessage(AbstractSession.java:555)
> 	org.apache.sshd.common.session.helpers.AbstractSession.decode(AbstractSession.java:1527)
> 	org.apache.sshd.common.session.helpers.AbstractSession.messageReceived(AbstractSession.java:516)
> 	org.apache.sshd.common.session.helpers.AbstractSessionIoHandler.messageReceived(AbstractSessionIoHandler.java:63)
> 	org.apache.sshd.common.io.nio2.Nio2Session.handleReadCycleCompletion(Nio2Session.java:339)
> 	org.apache.sshd.common.io.nio2.Nio2Session$1.onCompleted(Nio2Session.java:318)
> 	org.apache.sshd.common.io.nio2.Nio2Session$1.onCompleted(Nio2Session.java:315)
> 	org.apache.sshd.common.io.nio2.Nio2CompletionHandler.lambda$completed$0(Nio2CompletionHandler.java:38)
> 	org.apache.sshd.common.io.nio2.Nio2CompletionHandler$$Lambda$349/1690110070.run(Unknown
Source)
> 	java.security.AccessController.doPrivileged(Native Method)
> 	org.apache.sshd.common.io.nio2.Nio2CompletionHandler.completed(Nio2CompletionHandler.java:37)
> 	sun.nio.ch.Invoker.invokeUnchecked(Invoker.java:126)
> 	sun.nio.ch.Invoker$2.run(Invoker.java:218)
> 	sun.nio.ch.AsynchronousChannelGroupImpl$1.run(AsynchronousChannelGroupImpl.java:112)
> 	java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
> 	java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
> 	java.lang.Thread.run(Thread.java:748)
> "SSH git-upload-pack /project2 (myuser)" prio=1 BLOCKED
> 	org.apache.sshd.common.session.helpers.AbstractSession.writePacket(AbstractSession.java:1107)
> 	org.apache.sshd.common.channel.AbstractChannel.writePacket(AbstractChannel.java:798)
> 	org.apache.sshd.common.channel.ChannelOutputStream.flush(ChannelOutputStream.java:227)
> 	org.apache.sshd.common.channel.ChannelOutputStream.write(ChannelOutputStream.java:127)
> 	org.eclipse.jgit.transport.UploadPack$ResponseBufferedOutputStream.write(UploadPack.java:2183)
> 	org.eclipse.jgit.transport.SideBandOutputStream.writeBuffer(SideBandOutputStream.java:174)
> 	org.eclipse.jgit.transport.SideBandOutputStream.write(SideBandOutputStream.java:153)
> 	org.eclipse.jgit.internal.storage.pack.PackOutputStream.write(PackOutputStream.java:132)
> 	org.eclipse.jgit.internal.storage.file.PackFile.copyAsIs2(PackFile.java:614)
> 	org.eclipse.jgit.internal.storage.file.PackFile.copyAsIs(PackFile.java:433)
> 	org.eclipse.jgit.internal.storage.file.WindowCursor.copyObjectAsIs(WindowCursor.java:221)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjectImpl(PackWriter.java:1644)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObject(PackWriter.java:1621)
> 	org.eclipse.jgit.internal.storage.pack.PackOutputStream.writeObject(PackOutputStream.java:171)
> 	org.eclipse.jgit.internal.storage.file.WindowCursor.writeObjects(WindowCursor.java:229)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjects(PackWriter.java:1609)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjects(PackWriter.java:1597)
> 	org.eclipse.jgit.internal.storage.pack.PackWriter.writePack(PackWriter.java:1154)
> 	org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:2133)
> 	org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:1947)
> 	org.eclipse.jgit.transport.UploadPack.service(UploadPack.java:971)
> 	org.eclipse.jgit.transport.UploadPack.upload(UploadPack.java:776)
> 	com.google.gerrit.sshd.commands.Upload.runImpl(Upload.java:77)
> 	com.google.gerrit.sshd.AbstractGitCommand.service(AbstractGitCommand.java:98)
> 	com.google.gerrit.sshd.AbstractGitCommand.access$000(AbstractGitCommand.java:31)
> 	com.google.gerrit.sshd.AbstractGitCommand$1.run(AbstractGitCommand.java:63)
> 	com.google.gerrit.sshd.BaseCommand$TaskThunk.run(BaseCommand.java:467)
> 	com.google.gerrit.server.logging.LoggingContextAwareRunnable.run(LoggingContextAwareRunnable.java:83)
> 	java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
> 	java.util.concurrent.FutureTask.run(FutureTask.java:266)
> 	java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
> 	java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
> 	com.google.gerrit.server.git.WorkQueue$Task.run(WorkQueue.java:646)
> 	java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
> 	java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
> 	java.lang.Thread.run(Thread.java:748)
> {noformat}
> In AbstractSession.handleNewKeys(), there is a lock on {{pendingEvents}}, while trying
to send these packets; when sending the packets fails, an exception is generated, which causes
the channel to get closed by calling ChannelOutputStream.close(), which is a synchronized
method.
> At the same time, some other threads are trying to write data to the session: it calls
ChannelOutputStream.write(), which is also a synchronized method, which calls AbstractSession.writePacket()
which attempts to lock {{pendingPackets}} to queue the packets.
> *NOTE*: in our setup, we can reproduce this quite easily by increasing the parallelism
in git-repo, reducing the values of waitTimeout (WAIT_FOR_SPACE_TIMEOUT), rekeyBytesLimit
(REKEY_TIME_LIMIT) and rekeyTimeLimit (REKEY_TIME_LIMIT) configuration options.



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