diff --git a/synapse/util/async.py b/synapse/util/async.py
index bb252f75d7..0fd5b42523 100644
--- a/synapse/util/async.py
+++ b/synapse/util/async.py
@@ -203,7 +203,26 @@ class Linearizer(object):
except:
logger.exception("Unexpected exception in Linearizer")
- logger.info("Acquired linearizer lock %r for key %r", self.name, key)
+ logger.info("Acquired linearizer lock %r for key %r", self.name,
+ key)
+
+ # if the code holding the lock completes synchronously, then it
+ # will recursively run the next claimant on the list. That can
+ # relatively rapidly lead to stack exhaustion. This is essentially
+ # the same problem as http://twistedmatrix.com/trac/ticket/9304.
+ #
+ # In order to break the cycle, we add a cheeky sleep(0) here to
+ # ensure that we fall back to the reactor between each iteration.
+ #
+ # (There's no particular need for it to happen before we return
+ # the context manager, but it needs to happen while we hold the
+ # lock, and the context manager's exit code must be synchronous,
+ # so actually this is the only sensible place.
+ yield run_on_reactor()
+
+ else:
+ logger.info("Acquired uncontended linearizer lock %r for key %r",
+ self.name, key)
@contextmanager
def _ctx_manager():
@@ -211,7 +230,8 @@ class Linearizer(object):
yield
finally:
logger.info("Releasing linearizer lock %r for key %r", self.name, key)
- new_defer.callback(None)
+ with PreserveLoggingContext():
+ new_defer.callback(None)
current_d = self.key_to_defer.get(key)
if current_d is new_defer:
self.key_to_defer.pop(key, None)
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