From 65bb9b5ae78ee29100ea6bf5617b0fdbc0a100bd Mon Sep 17 00:00:00 2001
From: "simonmar@microsoft.com" <unknown>
Date: Thu, 28 Feb 2008 15:23:32 +0000
Subject: [PATCH] Updating a thunk in raiseAsync might encounter an IND; cope
 There was already a check to avoid updating an IND, but it
 was originally there to avoid a bug which doesn't exist
 now.  Furthermore the test and update are not atomic, so
 another thread could be updating this thunk while we are. 
 We have to just go ahead and update anyway - it might waste
 a little work, but this is a very rare case.

---
 rts/RaiseAsync.c |   21 ++++++---------------
 rts/Updates.h    |    5 +++--
 2 files changed, 9 insertions(+), 17 deletions(-)

diff --git a/rts/RaiseAsync.c b/rts/RaiseAsync.c
index ee53e0d..21bc78e 100644
--- a/rts/RaiseAsync.c
+++ b/rts/RaiseAsync.c
@@ -944,21 +944,12 @@ raiseAsync(Capability *cap, StgTSO *tso, StgClosure *exception,
 	    //	     printObj((StgClosure *)ap);
 	    //	);
 
-	    // Replace the updatee with an indirection
-	    //
-	    // Warning: if we're in a loop, more than one update frame on
-	    // the stack may point to the same object.  Be careful not to
-	    // overwrite an IND_OLDGEN in this case, because we'll screw
-	    // up the mutable lists.  To be on the safe side, don't
-	    // overwrite any kind of indirection at all.  See also
-	    // threadSqueezeStack in GC.c, where we have to make a similar
-	    // check.
-	    //
-	    if (!closure_IND(((StgUpdateFrame *)frame)->updatee)) {
-		// revert the black hole
-		UPD_IND_NOLOCK(((StgUpdateFrame *)frame)->updatee,
-			       (StgClosure *)ap);
-	    }
+            // Perform the update
+            // TODO: this may waste some work, if the thunk has
+            // already been updated by another thread.
+            UPD_IND_NOLOCK(((StgUpdateFrame *)frame)->updatee,
+                           (StgClosure *)ap);
+
 	    sp += sizeofW(StgUpdateFrame) - 1;
 	    sp[0] = (W_)ap; // push onto stack
 	    frame = sp + 1;
diff --git a/rts/Updates.h b/rts/Updates.h
index 3461c91..c04005b 100644
--- a/rts/Updates.h
+++ b/rts/Updates.h
@@ -280,8 +280,9 @@ no_slop:
   {								\
     bdescr *bd;							\
 								\
-    /* cas(p1, 0, &stg_WHITEHOLE_info); */			\
-    ASSERT( (P_)p1 != (P_)p2 && !closure_IND(p1) );		\
+    ASSERT( (P_)p1 != (P_)p2 );                                 \
+    /* not necessarily true: ASSERT( !closure_IND(p1) ); */     \
+    /* occurs in RaiseAsync.c:raiseAsync() */                   \
     DEBUG_FILL_SLOP(p1);					\
     LDV_RECORD_DEAD_FILL_SLOP_DYNAMIC(p1);			\
     ((StgInd *)p1)->indirectee = p2;				\
-- 
1.7.10.4