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https://issues.apache.org/jira/browse/DIRMINA-1107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16839167#comment-16839167
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Emmanuel Lecharny commented on DIRMINA-1107:
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What I mean is that the {{messageReceivedEventQueue}} is only fed by the thread
that read data from the channel (ie the {{IoProcessor}} thread). There is no
other thread that interact with it, and there is no reason to postpone the
propagation of the message to the chain. In other words, there is no reason to
process those messages in the {{flushScheduledEvents}} method.
The tricky part is to handle properly the handshake messages which should *not*
be propagated to the chain.
Anyway, that leave the other queue alone, and we can now safely process it in
the {{flushScheduledEvents}} method without any interaction with the read
messages.
I have to do some experiment around this idea.
> SslHandler flushScheduledEvents race condition, redux
> -----------------------------------------------------
>
> Key: DIRMINA-1107
> URL: https://issues.apache.org/jira/browse/DIRMINA-1107
> Project: MINA
> Issue Type: Bug
> Affects Versions: 2.1.2
> Reporter: Guus der Kinderen
> Priority: Major
> Fix For: 2.1.3
>
>
> DIRMINA-1019 addresses a race condition in SslHandler, but unintentionally
> replaces it with another multithreading issue.
> The fix for DIRMINA-1019 introduces a counter that contains the number of
> events to be processed. A simplified version of the code is included below.
> {code:java}
> private final AtomicInteger scheduledEvents = new AtomicInteger(0);
> void flushScheduledEvents() {
> scheduledEvents.incrementAndGet();
> if (sslLock.tryLock()) {
> try {
> do {
> while ((event = filterWriteEventQueue.poll()) != null) {
> // ...
> }
>
> while ((event = messageReceivedEventQueue.poll()) != null){
> // ...
> }
> } while (scheduledEvents.decrementAndGet() > 0);
> } finally {
> sslLock.unlock();
> }
> }
> }{code}
> We have observed occasions where the value of {{scheduledEvents}} becomes a
> negative value, while at the same time {{filterWriteEventQueue}} go
> unprocessed.
> We suspect that this issue is triggered by a concurrency issue caused by the
> first thread decrementing the counter after a second thread incremented it,
> but before it attempted to acquire the lock.
> This allows the the first thread to empty the queues, decrementing the
> counter to zero and release the lock, after which the second thread acquires
> the lock successfully. Now, the second thread processes any elements in
> {{filterWriteEventQueue}}, and then processes any elements in
> {{messageReceivedEventQueue}}. If in between these two checks yet another
> thread adds a new element to {{filterWriteEventQueue}}, this element can go
> unprocessed (as the second thread does not loop, since the counter is zero or
> negative, and the third thread can fail to acquire the lock).
> It's a seemingly unlikely scenario, but we are observing the behavior when
> our systems are under high load.
> We've applied a code change after which this problem is no longer observed.
> We've removed the counter, and check on the size of the queues instead:
> {code:java}
> void flushScheduledEvents() {
> if (sslLock.tryLock()) {
> try {
> do {
> while ((event = filterWriteEventQueue.poll()) != null) {
> // ...
> }
>
> while ((event = messageReceivedEventQueue.poll()) != null){
> // ...
> }
> } while (!filterWriteEventQueue.isEmpty() ||
> !messageReceivedEventQueue.isEmpty());
> } finally {
> sslLock.unlock();
> }
> }
> }{code}
> This code change, as illustrated above, does introduce a new potential
> problem. Theoretically, an event could be added to the queues and
> {{flushScheduledEvents}} be called returning {{false}} for
> {{sslLock.tryLock()}}, exactly after another thread just finished the
> {{while}} loop, but before releasing the lock. This again would cause events
> to go unprocessed.
> We've not observed this problem in the wild yet, but we're uncomfortable
> applying this change as-is.
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