diff --git a/master.cfg b/master.cfg
index 4378767..08bfdea 100644
--- a/master.cfg
+++ b/master.cfg
@@ -600,6 +600,9 @@ class RunTestGDBIndexBuildBSD (RunTestGDBIndexBuild, RunTestGDBBSD_Common):
 # For now, we only support testing the "master" branch.
 all_gdb_filter = ChangeFilter (branch_fn = should_watch_branch)
 
+def DefaultGDBCanStartBuild (builder, buildslave, buildrequest):
+    return not builder.building
+
 ###############################
 #### Configuration loading ####
 ###############################
@@ -640,6 +643,7 @@ def load_config (c):
         btype = b.pop ('type')
         factory = globals ()[ "RunTestGDB%s" % btype ]
         b['factory'] = factory (arch_triplet)
+        b['canStartBuild'] = DefaultGDBCanStartBuild
         c['builders'].append (b)
 
 load_config (c)