Add changelog entry

changelog.d/18124.misc

@@ -0,0 +1 @@
+Add log message when worker lock timeouts get large.

synapse/handlers/worker_lock.py

@@ -19,6 +19,7 @@
 #
 #
 
+import logging
 import random
 from types import TracebackType
 from typing import (
@@ -269,6 +270,10 @@ class WaitingLock:
     def _get_next_retry_interval(self) -> float:
         next = self._retry_interval
         self._retry_interval = max(5, next * 2)
+        if self._retry_interval > 5 * 2 ^ 7:  # ~10 minutes
+            logging.warning(
+                f"Lock timeout is getting excessive: {self._retry_interval}s. There may be a deadlock."
+            )
         return next * random.uniform(0.9, 1.1)
 
 
@@ -344,4 +349,8 @@ class WaitingMultiLock:
     def _get_next_retry_interval(self) -> float:
         next = self._retry_interval
         self._retry_interval = max(5, next * 2)
+        if self._retry_interval > 5 * 2 ^ 7:  # ~10 minutes
+            logging.warning(
+                f"Lock timeout is getting excessive: {self._retry_interval}s. There may be a deadlock."
+            )
         return next * random.uniform(0.9, 1.1)