LakshSingla opened a new pull request, #16372: URL: https://github.com/apache/druid/pull/16372
### Description This PR fixes up a deadlock that was introduced by https://github.com/apache/druid/pull/15420. Consider the following sequence of calls: | Time | Thread1 | Thread2 | |--------|--------|--------| | 1 | pool.reserve(query1) | | | 2 | | pool.reserve(query2) | | 3 | pool.clean(query1) | | Assuming that the pool has enough merge buffers to allow each query to proceed individually, but not together, the following will happen (in the original code). The `clean(at t=3, by thread1)` would be blocked on the `reserve(at t=2, by thread2)` to complete. However, the reserve won't succeed since it doesn't have enough merge buffers, and is waiting on the clean call, hence the cyclic dependency. (**NOTE**: This also assumes that the resourceId of query1 and query2 would hold the lock on the same entry in the concurrent hash map). The fix is to reserve the buffer before acquiring a lock on the concurrent hash map and hold the locks for the minimal time possible. This way, other calls like `remove()` and `fetch()` are not blocked on the actual acquisition of the merge buffers to succeed. I have added a test case to replicate the sequence of events. The test case fails before the patch, however succeeds after the patch. The test case isn't perfect, however, given the blocking nature of the reserve() call, I couldn't find an alternative. #### Release note <hr> ##### Key changed/added classes in this PR * `MyFoo` * `OurBar` * `TheirBaz` <hr> <!-- Check the items by putting "x" in the brackets for the done things. Not all of these items apply to every PR. Remove the items which are not done or not relevant to the PR. None of the items from the checklist below are strictly necessary, but it would be very helpful if you at least self-review the PR. --> This PR has: - [ ] been self-reviewed. - [ ] using the [concurrency checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md) (Remove this item if the PR doesn't have any relation to concurrency.) - [ ] added documentation for new or modified features or behaviors. - [ ] a release note entry in the PR description. - [ ] added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links. - [ ] added or updated version, license, or notice information in [licenses.yaml](https://github.com/apache/druid/blob/master/dev/license.md) - [ ] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader. - [ ] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for [code coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md) is met. - [ ] added integration tests. - [ ] been tested in a test Druid cluster. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
