8029281: Synchronization issues in Logger and LogManager

[Daniel Fuchs]

On 11/27/13 10:34 PM, Peter Levart wrote:
> Hi Daniel,
>
> I have started looking at the LogManager change first, and here's the
> 1st race I found...

Hi Peter,

Thanks a lot for your feedback! see below...

>   The new method LoggerWeakRef.dispose() can be called from 3 places:
>
> - LoggerContext.addLocalLogger
> - LoggerContext.findLogger
> - LogManager.drainLoggerRefQueueBounded which is called from public
> method LogManager.addLogger
>
> The 1st and 2nd are guarded by LoggerContext.this lock, but the 3rd is
> unguarded.
>
> What can happen is the following:
>
> Thread1:
>      LogManager.addLogger(someLogger)
>          LogManager.drainLoggerRefQueueBounded() // finds some enqueued
> LoggerWeakRef for name == 'X.Y' and...
>              LoggerWeakRef.dispose() // this is 1st call to dispose, so...
>                  synchronized (LoggerWeakRef.this) {
>                     disposed = true;
>                  }
>
> ---context switch---
>
> Thread2:
>          LogManager.addLogger(logger{name=='X.Y'})
>              LogManager.drainLoggerRefQueueBounded()  // no enqueued refs...
>              LoggerContext.addLocalLogger(logger{name=='X.Y'}, true)
>                  LoggerWeakRef ref = namedLoggers.get(name); // returns
> a non-null ref ('X.Y' still points to a cleared weak ref)
>                  if (ref.get() == null) {
>                      ref.dispose(); // this is a 2nd call to this ref,
> so it returns quickly
>                  }
>                  // ... namedLoggers gets new entry, etc...
>                  LogNode node = getNode(name); // get node for 'X.Y'
> *                node.loggerRef = ref;*
>
> ---context switch---
>
> Thread1: (still in LoggerWeakRef.dispose())
>              if (node != null) {
> *                node.loggerRef = null;*  // clear LogNode's weak ref to
> us *!!! NOT QUITE !!!*
>
>
> ... so Thread1 erroneously clears the reference from Node to new
> LoggerWeakRef that was just added by Thread2 ...

Damn. I believe that you're right! I overlooked the fact that LogNode is
reused.

Locking on LogNode.context is indeed a good way of solving the issue.
The only method that we will call from within the lock is 
LogNode.context.rmoveLoggerRef(name this) - and that already requires
a lock on LogNode.context so it would be safe.

I have also double checked the places where LogNode.loggerRef is set,
and all those places (except in dispose() so far) are already from
within a lock on LoggerContext.

I think I would prefer however to keep the 'disposed' flag - and to
check whether LogNode.loggerRef == this before clearing it.

Given that - as I said - LogNode.loggerRef is always modified
from within a lock on LoggerContext I think the following would
work:

void dispose() {
     synchronized(this) {
         if (disposed) return;
         disposed = true;
     }

     final LogNode n = node;
     if (n != null) {
         synchronized (n.context) {
             n.context.removeLoggerRef(name, this);
             name = null;
             if (n.loggerRef == this) {
                 n.loggerRef = null;
             }
             node = null;
          }
     }

     if (parentRef != null) {
         Logger parent = parentRef.get();
         if (parent != null) {
             parent.removeChildLogger(this);
         }
         parentRef = null;  // clear our weak ref to the parent Logger
     }
}

I don't think 'node' or 'parentRef' need to be volatile.
That - said - we could still declare them volatile to make it obvious
that the code in 'dispose()' will not see a stale value (I don't
think it can, but having the variables volatile would make it
obvious).
If we reach 'dispose()' then it does mean that the LoggerWeakRef
is no longer used - so I believe the real thing we need to guard against
is actually modifications of the LogNode state - rather modification
of the LoggerWeakRef  state.
Therefore I believe that the important thing is to guarantee that
all modifications to LogNode.loggerRef happen from within a lock
on LoggerContext.

I'd also like to keep the disposed flag because it makes it immediately
obvious that the method is processed only once (so we haven't to worry
about making it re-entrant at every step).

Would you agree with the code above?

best regards, and congratulations for catching this!

-- daniel

>
>
> One way to fix this is to synchronize the clearing of node.logerRef on
> node.context too. I would make LoggerWeakRef.node and parentRef fields
> volatile and do the following:
>
>
>      final class LoggerWeakRef extends WeakReference<Logger> {
>          private String                         name; // for
> namedLoggers cleanup
>          private volatile LogNode               node; // for loggerRef
> cleanup
>          private volatile WeakReference<Logger> parentRef;  // for kids
> cleanup
>
>          LoggerWeakRef(Logger logger) {
>              super(logger, loggerRefQueue);
>              name = logger.getName();  // save for namedLoggers cleanup
>          }
>
>          // dispose of this LoggerWeakRef object
>          void dispose() {
>              LogNode node = this.node;
>              if (node != null) {
>                  synchronized (node.context) {
>                      node = this.node; // double-checked locking
>                      if (node != null) {
>                          // if we have a LogNode, then we were a named
> Logger
>                          // so clear namedLoggers weak ref to us
>                          node.context.removeLoggerRef(name, this);
>                          name = null;  // clear our ref to the Logger's name
>
>                          node.loggerRef = null;  // clear LogNode's weak
> ref to us
>                          this.node = null;       // clear our ref to LogNode
>                      }
>                  }
>              }
>
>              WeakReference<Logger> parentRef = this.parentRef;
>              if (parentRef != null) {
>                  // this LoggerWeakRef has or had a parent Logger
>                  this.parentRef = null;  // clear our weak ref to the
> parent Logger (racy, but harmless)
>                  Logger parent = parentRef.get();
>                  if (parent != null) {
>                      // the parent Logger is still there so clear the
>                      // parent Logger's weak ref to us
>                      parent.removeChildLogger(this);
>                  }
>              }
>          }
>
> ...
>
>
> What do you think?
>
> That's all for today. Will check the rest of patch tomorrow.
>
>
> Regards, Peter
>
>
>
> On 11/27/2013 08:23 PM, Daniel Fuchs wrote:
>> Hi,
>>
>> Please find below a webrev for:
>>
>> 8029281: Synchronization issues in Logger and LogManager
>> <https://bugs.openjdk.java.net/browse/JDK-8029281:
>>
>> I believe this changeset will also fix JDK-8027670 and
>> JDK-8029092 - which I have thus closed as duplicates of JDK-8029281.
>>
>> webrev: <http://cr.openjdk.java.net/~dfuchs/webrev_8029281/webrev.00/>
>>
>> Now some comments on the fix:
>>
>> LogManager:
>>
>> When a logger was garbage collected, and then a new logger
>> of the same name requested, addlocalLogger used to call
>> LoggerContext.removeLogger to remove the stale reference
>> and replace it by a new one. But sometime, the stale reference
>> was still in the reference queue, which caused the *new* logger
>> to be wrongly removed later when the reference queue was drained.
>>
>> LogManager was also missing some synchronization for its 'props'
>> variable.
>>
>> Logger:
>>
>> userBundle & resourceBundleName were sometimes accessed within
>> a synchronized block, and sometimes without. In particular the
>> getters were not synchronized, which could cause race conditions
>> because an other thread could see inconsistent data.
>>
>> I also had to refactor the two methods getEffectiveResourceBundle()
>> and getResourceBundleName() into a single method.
>> The code there might be a bit more difficult to follow,
>> because I made sure it preserved the former behavior wrt to
>> what will be set in the LogRecord in doLog().
>>
>> Tests:
>>
>> The new TestLoggerBundleSync.java has been great to test
>> the issues above in both Logger & LogManager (it detected
>> the drainLoggerRefQueueBounded issue).
>>
>> Since I had to touch at drainLoggerRefQueueBounded to make
>> TestLoggerBundleSync.java pass I also threw in a
>> TestLogConfigurationDeadLock.java to make sure I hadn't introduced
>> any new deadlock (and it detected the lack of synchronization
>> in LogManager.props).
>>
>> best regards,
>>
>> -- daniel
>>
>>
>>
>