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From "Michael Cooper (JIRA)" <j...@apache.org>
Subject [jira] Reopened: (AMQ-2475) If tmp message store fills up, broker can deadlock due to while producers wait on disk space and consumers wait on acks
Date Thu, 17 Jun 2010 06:10:52 GMT

     [ https://issues.apache.org/activemq/browse/AMQ-2475?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Michael Cooper reopened AMQ-2475:
---------------------------------


I am still able to consistently see this deadlock behavior on our system with both version
5.3.0 and 5.3.2.  The patch loops in the TopicSubscription add method while holding the matchedListMutex
until thePendingMessageCursor "matched" is not full.  The code then attempts to call addMessageLast
on the "matched" instance assuming that the matchedListMutex will prevent any additional threads
from taking that space.  This assumption is wrong, because I have found that when using a
filePendingMessageCursor, the addMessageLast method will end up calling systemUsage.getTempUsage().waitForSpace()
which for whatever reason can be full when it is called and without the ability to reduce
in size due to monitors already held earlier in the stack.  Therefore, the code loops infinitely
and the system is deadlocked.

To workaround this issue, I switched to using vm cursors, which don't rely on this shared
pool of temp file storage, and haven't seen the deadlock.

I am new to this project and still trying to understand the code completely, but this is what
I have found.  I think the looping that is happening to wait for space is happening to early
in the stack.  The matchedListMutex does not seem to lock out other threads that use temp
storage.  I'm not sure what the correct fix is, but without a significant reworking of the
code, the best I can think to do would be to have the addMessageLast method throw some kind
of exception or have a return value if space is not available, so the calling method can again
release its the matchedListMutext by calling wait, and try again.  And addMessageLast wold
also not call waitForSpace with an infinite timeout, but instead specify a small timeout.

> If tmp message store fills up, broker can deadlock due to while producers wait on disk
space and consumers wait on acks
> -----------------------------------------------------------------------------------------------------------------------
>
>                 Key: AMQ-2475
>                 URL: https://issues.apache.org/activemq/browse/AMQ-2475
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: Broker, Message Store, Transport
>    Affects Versions: 5.3.0
>         Environment: Tested on Windows XP with JDK 1.60_13, but fairly sure it will be
an issue on all platforms
>            Reporter: Martin Murphy
>            Assignee: Rob Davies
>             Fix For: 5.3.1, 5.4.0
>
>         Attachments: activemq.xml, hangtest.zip, Queue.java, Queue.patchfile.txt, Topic.java,
Topic.patchfile.txt, TopicSubscription.java, TopicSubscription.patchfile.txt
>
>
> I will attach a simple project that shows this. In the test the tmp space is set to 32
MB and two threads are created. One thread will constantly produce 1KB messages and the other
consumes these, but sleeps for 100ms, note that producer flow control is turned off as well.
The goal here is to ensure that the producers block while the consumers read the rest of the
messages from the broker and catch up, this in turn frees up the disk space and allows the
producer to send more messages. This config means that you can bound the broker based on disk
space rather than memory usage.
> Unfortunately in this test using topics while the broker is reading in the message from
the producer it has to lock the matched list it is adding it to. This is an abstract from
the Topic's point of view and doesn't realize that the file may block based on the file system.

> {code}
>     public void add(MessageReference node) throws Exception { //... snip ...
>             if (maximumPendingMessages != 0) {
>                 synchronized (matchedListMutex) {   // We have this mutex
>                     matched.addMessageLast(node); // ends up waiting for space
>                     // NOTE - be careful about the slaveBroker!
>                     if (maximumPendingMessages > 0) {
> {code}
> Meanwhile the consumer is sending acknowledgements for the 10 messages it just read in
(the configured prefetch) from the same topic, but since they also modify the same list in
the topic this waits as well on the mutex held to service the producer:
> {code}
>     private void dispatchMatched() throws IOException {       
>         synchronized (matchedListMutex) {  // never gets passed here.
>             if (!matched.isEmpty() && !isFull()) {
> {code}
> This is a fairly classic deadlock. The trick is now how to resolve this given the fact
that the topic isn't aware that it's list may need to wait for the file system to clean up.

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