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https://issues.apache.org/jira/browse/DIRMINA-1107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16838898#comment-16838898
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Emmanuel Lecharny commented on DIRMINA-1107:
--------------------------------------------
There are a few bad things done. In the {{SslFilter.messageReceived()}} method,
we push any incoming message if the {{Ssl}} layer has not yet been installed
fully :
{code:java}
synchronized (sslHandler) {
if (!isSslStarted(session) && sslHandler.isInboundDone()) {
// The SSL session must be established first before we
// can push data to the application. Store the incoming
// data into a queue for a later processing
sslHandler.scheduleMessageReceived(nextFilter, message);
} else {
{code}
It does not make a lot of sense. There is absolutely no way the {{SslFilter}}
{{messageReceived}} method could be called if the filter has not been set
fully. The check done is for the case where the {{SSL}} layer has been closed,
but in this very case, what's the point in passing the received message to the
{{IoHandler}} ?
Otherwise, incoming messages are processed in the
{{SslHandler.messageReceived}} method, and the message will be uncrypted, then
pushed into the queue. The bizarre thing is that we do write encrypted data
during this call :
{code:java}
private void handleSslData(NextFilter nextFilter, SslHandler sslHandler)
throws SSLException {
...
// Write encrypted data to be written (if any)
sslHandler.writeNetBuffer(nextFilter);
// handle app. data read (if any)
handleAppDataRead(nextFilter, sslHandler);
}
{code}
I don't see any reason to do so at this point. Why should we write *anything*
back to the remote peer when we are processing an incoming message ?
This part of the code needs a serious review, IMHO.
> 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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