"Memory leak" caused by WorkQueue#topLevelExec

[Olivier Peyrusse]

Hello community,

Sorry if this is the wrong place to discuss internal classes such as the ForkJoinPool. If so, please, excuse me and point me in the right direction.

At my company, we have experienced an unfortunate memory leak because one of our CountedCompleter was retaining a large object and the task was not released to the GC (I will give more details below but will first focus on the FJP code causing the issue).

When running tasks, the FJP ends up calling [WorkQueue#topLevelExec](https://github.com/openjdk/jdk/blob/c419dda4e99c3b72fbee95b93159db2e23b994b6/src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java#L1448-L1453), which is implemented as follow:

final void topLevelExec(ForkJoinTask<?> task, int fifo) {
while (task != null) {
task.doExec();
task = nextLocalTask(fifo);
} }

We can see that it starts from a top-level task task​, executes it, and looks for the next task to execute before repeating this loop. This means that, as long as we find a task through nextLocalTask​​, we do not exit this method and the caller of topLevelExec​ retains in its stack a reference to the first executed task - like [here](https://github.com/openjdk/jdk/blob/c419dda4e99c3b72fbee95b93159db2e23b994b6/src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java#L1992-L2019). This acts as a path from the GC root, preventing the garbage collection of the task.
So even if a CountedCompleter did complete its exec / tryComplete / etc, the framework will keep the object alive.
Could the code be changed to avoid this issue? I am willing to do the work, as well as come up with a test case reproducing the issue if it is deemed needed.

In our case, we were in the unfortunate situation where our counted completer was holding an element which happened to be a sort of head of a dynamic sort of linked queue. By retaining it, the rest of the growing linked queue was also retained in memory, leading to the memory leak.
Obvious fixes are possible in our code, by ensuring that we nullify such elements when our operations complete, and more ideas. But this means that we have to be constantly careful about the fields we pass to the task, what is captured if we give lambdas, etc. If the whole ForkJoinPool could also be improved to avoid such problems, it would be an additional safety.

Thank you for reading the mail
Cheers

Olivier
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