[logback-user] Best practices to avoid deadlocks while logging

[Chris Miller]

What's happening here, as I guess you're aware, is that multiple locks are 
not being aquired in the same fixed order globally - a situation prone to 
deadlocking. But on top of that, Logback is unintentially executing a denial-of-service 
attack on your app by locking the Worker instance. The reason this can happen 
is because your Worker code is synchronizing on the Worker object itself. 
That object is exposed to the outside world and therefore you have no control 
over who else can lock on it and therefore no way to prevent possible deadlocks 
or a plain denial-of-service.

Best practices is to never use "synchronized(this) { ... }" or synchronize 
entire methods (which amounts to the same thing) in your code. Instead your 
Worker class should have a private lock object of its own that you synchronize 
on. This makes the lock object inaccessible to the outside world and therefore 
impossible (without malicious reflection at least!) to cause a denial-of-service 
situation like you are experiencing.

eg:

public class Worker {
  private static final Logger logger = LoggerFactory.getLogger(Worker.class);
  private final Object LOCK = new Object();

  public void work() {
    synchronized(LOCK) {
      ...
      logger.info("Did some work...");
    }
    ...
}

Note that the 'final' is essential for the LOCK object declaration (and really 
for the logger variable too). Without final, you risk different threads locking 
on different objects and visibility issues between threads. In your implementation 
of getStatus() { ... } you declare a final StringBuffer though - I guess 
you're aware that doesn't have any impact on the threadsafety of your code?


> Dear list,
> 
> I recently had a deadlock problem related to logging and I am
> wondering if it comes from a problem in the logging framework (I must
> say I doubt it) or from the way I use logging, in which case I'd like
> to hear advices about how others avoid this kind of problems.
> 
> The pattern is the following. (Please see hereby the java classes
> which
> might be clearer.)
> - In a class "Worker", a method "work()" locks the object instance of
> the class Worker to do some work and calls the logging framework to
> print the result (while still holding the lock).
> - In the same class "Worker", an other method "getStatus()" obtains
> the
> current status of the object instance of the class Worker and returns
> it
> as a String (method used for logging). That method locks the object at
> some point to make sure it reads consistent internal status
> information.
> - In an other class "Accessor", a method "access()" performs some work
> (not locking anything), then logs the status of an object of the class
> Worker by using its getStatus() method.
> Now the deadlock scenario is the following.
> - A thread "WorkerThread" is executing the method "work()". It has
> acquired the lock for the Worker object and has not reached the
> logging
> statement yet.
> - An other thread "AccessorThread" is executing the method "access()",
> and is executing the logging statement.
> AFAIU, logback console logger has some sort of "central" locking
> point:
> when a thread is logging, an other thread may not log in parallel
> (that
> seems good as the user does not want his different logging statements
> to
> be mixed together)... Thus, the following happens.
> - AccessorThread has a lock to the Logger, and waits to acquire the
> Worker lock to execute the synchronized block in getStatus().
> - WorkerThread is holding that precise lock, and will never release
> it,
> because arriving at the logging statement in "work()", it starts
> waiting
> for the Logger lock acquired by AccessorThread. These two threads are
> caught in a deadlock.
> I feel the problem comes from not respecting the general rule that a
> block of code holding a lock should not call a method on an externally
> controlled object (possibly shared with other threads) whose locking
> behavior is unknown. (Note that both my code and the logging framework
> do not respect this rule, AFAIK.) So I should not have issued the log
> statement in my synchronized block in "work()". In my case, I can
> remove this logging statement from the synchronized block, but it is
> not clear to me whether it is always possible (or easy) to do so,
> especially if the thing you want to log depends on an internal state
> which might change as soon as you release the lock.
> 
> An other way to solve that problem is to not use the parametrized
> message system in SLF4J. Indeed, it would disable the possibility of
> the AccessorThread to try to acquire the lock on Worker while being in
> the SLF4J method.
> 
> I am wondering however if there is not a way that slf4j implementation
> (logback) release the lock on its logger, then gets the Worker lock,
> then re-acquire the lock on its logger only after having released the
> Worker lock, so that it has only one lock at a time? I tend to think
> that it would enable a user of the logging framework to pay no
> attention of the deadlock possibilities incurred by the logging
> framework, and to avoid him introducing subtle bugs with logging
> statements... But I may be wrong and I am not an expert in multithread
> programming.
> 
> More generally speaking, what would be the advices from experienced
> programmers to avoid this problem? How do you log in a multithreading
> context?
> 
> Also I am wondering if that (anti-)pattern could be documented on the
> logback page, to avoid others to do the same mistake as me? Or even on
> the SLF4J page, if it is not logback-specific?
> 
> The attached classes provide a test case showing the deadlock
> scenario. The test fails reliably on my box.
> 
> Thank you for any help.
> Olivier