Reentrant logging deadlock causes worker termination with SIGKILL

Gunicorn versions < 20.1 experience a RuntimeError when multiple worker processes attempt to write to stderr simultaneously, triggering reentrant or recursive logging that causes logging deadlocks. Workers are forcefully killed with signal 9 (SIGKILL), leading to intermittent application downtime despite normal CPU and memory usage.

Upgrade Gunicorn to version 21.2.0 or later (23.0.0 recommended). Replace direct stderr logging with logging.StreamHandler(sys.stdout) in Python code. Configure max_requests (e.g., 1000) and max_requests_jitter (e.g., 100) to recycle workers periodically. Consider reducing worker count during peak concurrency. Monitor stderr and worker signals even when application logs appear normal.