Do not segfault if no common root can be found

When trying to profile a plugin, ghc mysteriously segfaulted.  Upon
closer examination the segfault happend due to a `->prevStack` lookup
on a NULL pointer.

A new CostCentre: Unknown is introduced that is set, if ccsapp and
ccsfn are of equal depth (e.g. 0), and do not have a common CostCentre
in their stacks.

Reviewers: bgamari, simonmar, austin, erikd

Reviewed By: simonmar

Subscribers: Phyx, thomie

Differential Revision: https://phabricator.haskell.org/D2551

1 files changed, 5 insertions, 0 deletions

diff --git a/rts/Linker.c b/rts/Linker.c
index f16fb83996..3eeb46eae1 100644
--- a/rts/Linker.c
+++ b/rts/Linker.c
@@ -1399,6 +1399,11 @@ static SymbolAddr* lookupSymbol_ (SymbolName* lbl)
                 errorBelch("Could not on-demand load symbol '%s'\n", lbl);
                 return NULL;
             }
+#ifdef PROFILING
+            // collect any new cost centres & CCSs
+            // that were defined during runInit
+            initProfiling2();
+#endif
         }
 
         return val;