diff --git a/waflib/Runner.py b/waflib/Runner.py
index 7535c83d..975c3292 100644
--- a/waflib/Runner.py
+++ b/waflib/Runner.py
@@ -226,6 +226,10 @@ class Parallel(object):
 					pass
 				else:
 					if cond:
+						# The most common reason is conflicting build order declaration
+						# for example: "X run_after Y" and "Y run_after X"
+						# Another can be changing "run_after" dependencies while the build is running
+						# for example: updating "tsk.run_after" in the "runnable_status" method
 						lst = []
 						for tsk in self.postponed:
 							deps = [id(x) for x in tsk.run_after if not x.hasrun]
@@ -298,6 +302,8 @@ class Parallel(object):
 	def mark_finished(self, tsk):
 		def try_unfreeze(x):
 			# DAG ancestors are likely to be in the incomplete set
+			# This assumes that the run_after contents have not changed
+			# after the build starts, else a deadlock may occur
 			if x in self.incomplete:
 				# TODO remove dependencies to free some memory?
 				# x.run_after.remove(tsk)
diff --git a/waflib/Task.py b/waflib/Task.py
index ffe7b16e..fd7a453f 100644
--- a/waflib/Task.py
+++ b/waflib/Task.py
@@ -590,6 +590,9 @@ class Task(evil):
 		"""
 		Run this task only after the given *task*.
 
+		Calling this method from :py:meth:`waflib.Task.Task.runnable_status` may cause
+		build deadlocks; see :py:meth:`waflib.Tools.fc.fc.runnable_status` for details.
+
 		:param task: task
 		:type task: :py:class:`waflib.Task.Task`
 		"""
diff --git a/waflib/Tools/fc.py b/waflib/Tools/fc.py
index 621eb502..d9e8d8c4 100644
--- a/waflib/Tools/fc.py
+++ b/waflib/Tools/fc.py
@@ -121,6 +121,8 @@ class fc(Task.Task):
 		for k in ins.keys():
 			for a in ins[k]:
 				a.run_after.update(outs[k])
+				for x in outs[k]:
+					self.generator.bld.producer.revdeps[x].add(a)
 
 				# the scanner cannot output nodes, so we have to set them
 				# ourselves as task.dep_nodes (additional input nodes)