diff --git a/mps/code/mpm.h b/mps/code/mpm.h
index a031792ffc4..2ecef0b1852 100644
--- a/mps/code/mpm.h
+++ b/mps/code/mpm.h
@@ -658,6 +658,7 @@ extern Bool SegNext(Seg *segReturn, Arena arena, Seg seg);
 extern Bool SegNextOfRing(Seg *segReturn, Arena arena, Pool pool, Ring next);
 extern void SegSetWhite(Seg seg, TraceSet white);
 extern void SegSetGrey(Seg seg, TraceSet grey);
+extern void SegFlip(Seg seg, Trace trace);
 extern void SegSetRankSet(Seg seg, RankSet rankSet);
 extern void SegSetRankAndSummary(Seg seg, RankSet rankSet, RefSet summary);
 extern Res SegMerge(Seg *mergedSegReturn, Seg segLo, Seg segHi);
diff --git a/mps/code/mpmst.h b/mps/code/mpmst.h
index 57e0f4a24cc..c63ca4320f1 100644
--- a/mps/code/mpmst.h
+++ b/mps/code/mpmst.h
@@ -223,6 +223,7 @@ typedef struct SegClassStruct {
   SegSetBufferMethod setBuffer; /* set the segment buffer  */
   SegUnsetBufferMethod unsetBuffer; /* unset the segment buffer */
   SegSetGreyMethod setGrey;     /* change greyness of segment */
+  SegFlipMethod flip;           /* raise barrier for a flipped trace */
   SegSetWhiteMethod setWhite;   /* change whiteness of segment */
   SegSetRankSetMethod setRankSet; /* change rank set of segment */
   SegSetRankSummaryMethod setRankSummary; /* change rank set & summary */
diff --git a/mps/code/mpmtypes.h b/mps/code/mpmtypes.h
index 9d6e54c990e..15786d6244a 100644
--- a/mps/code/mpmtypes.h
+++ b/mps/code/mpmtypes.h
@@ -152,6 +152,7 @@ typedef void (*FreeBlockVisitor)(Addr base, Addr limit, Pool pool, void *p);
 typedef Res (*SegInitMethod)(Seg seg, Pool pool, Addr base, Size size,
                              ArgList args);
 typedef void (*SegSetGreyMethod)(Seg seg, TraceSet grey);
+typedef void (*SegFlipMethod)(Seg seg, Trace trace);
 typedef void (*SegSetWhiteMethod)(Seg seg, TraceSet white);
 typedef void (*SegSetRankSetMethod)(Seg seg, RankSet rankSet);
 typedef void (*SegSetRankSummaryMethod)(Seg seg, RankSet rankSet,
diff --git a/mps/code/seg.c b/mps/code/seg.c
index 3debf913d67..b5b2a46b76a 100644
--- a/mps/code/seg.c
+++ b/mps/code/seg.c
@@ -257,6 +257,20 @@ void SegSetGrey(Seg seg, TraceSet grey)
 }
 
 
+/* SegFlip -- update barriers for trace that's about to flip */
+
+void SegFlip(Seg seg, Trace trace)
+{
+  AVERT(Seg, seg);
+  AVERT(Trace, trace);
+
+  /* Don't dispatch to the class method unless the segment is grey for
+     the trace that's about to flip, and contains references. */
+  if (TraceSetIsMember(SegGrey(seg), trace) && SegRankSet(seg) != RankSetEMPTY)
+    Method(Seg, seg, flip)(seg, trace);
+}
+
+
 /* SegSetWhite -- change the whiteness of a segment
  *
  * Sets the segment whiteness to the trace set ts.
@@ -788,6 +802,16 @@ static void segNoSetGrey(Seg seg, TraceSet grey)
 }
 
 
+/* segTrivFlip -- ignore trace that's about to flip */
+
+static void segTrivFlip(Seg seg, Trace trace)
+{
+  AVERT(Seg, seg);
+  AVERT(Trace, trace);
+  AVER(seg->rankSet != RankSetEMPTY);
+}
+
+
 /* segNoSetWhite -- non-method to change the whiteness of a segment */
 
 static void segNoSetWhite(Seg seg, TraceSet white)
@@ -1216,6 +1240,29 @@ static void mutatorSegSetGrey(Seg seg, TraceSet grey)
   }
 }
 
+/* mutatorSegFlip -- update barriers for a trace that's about to flip */
+
+static void mutatorSegFlip(Seg seg, Trace trace)
+{
+  TraceSet flippedTraces;
+  Arena arena;
+
+  NextMethod(Seg, MutatorSeg, flip)(seg, trace);
+
+  /* Raise the read barrier if the segment was not grey for any
+     currently flipped trace. */
+  arena = PoolArena(SegPool(seg));
+  flippedTraces = arena->flippedTraces;
+  if (TraceSetInter(SegGrey(seg), flippedTraces) == TraceSetEMPTY) {
+    ShieldRaise(arena, seg, AccessREAD);
+  } else {
+    /* If the segment is grey for some currently flipped trace then
+       the read barrier must already have been raised, either in this
+       method or in mutatorSegSetGrey. */
+    AVER(SegSM(seg) & AccessREAD);
+  }
+}
+
 
 /* gcSegSetWhite -- GCSeg method to change whiteness of a segment
  *
@@ -1644,6 +1691,7 @@ DEFINE_CLASS(Seg, Seg, klass)
   klass->setBuffer = segNoSetBuffer;
   klass->unsetBuffer = segNoUnsetBuffer;
   klass->setGrey = segNoSetGrey;
+  klass->flip = segTrivFlip;
   klass->setWhite = segNoSetWhite;
   klass->setRankSet = segNoSetRankSet;
   klass->setRankSummary = segNoSetRankSummary;
@@ -1688,6 +1736,7 @@ DEFINE_CLASS(Seg, MutatorSeg, klass)
   INHERIT_CLASS(klass, MutatorSeg, GCSeg);
   klass->setSummary = mutatorSegSetSummary; 
   klass->setGrey = mutatorSegSetGrey;
+  klass->flip = mutatorSegFlip;
   klass->setRankSet = mutatorSegSetRankSet;
   klass->setRankSummary = mutatorSegSetRankSummary;
   AVERT(SegClass, klass);
diff --git a/mps/code/trace.c b/mps/code/trace.c
index 33f94efb55d..0a493e89bdb 100644
--- a/mps/code/trace.c
+++ b/mps/code/trace.c
@@ -590,15 +590,11 @@ static Res traceFlip(Trace trace)
   /* drj 2003-02-19) */
 
   /* Now that the mutator is black we must prevent it from reading */
-  /* grey objects so that it can't obtain white pointers.  This is */
-  /* achieved by read protecting all segments containing objects */
-  /* which are grey for any of the flipped traces. */
+  /* grey objects so that it can't obtain white pointers. */
   for(rank = RankMIN; rank < RankLIMIT; ++rank)
     RING_FOR(node, ArenaGreyRing(arena, rank), nextNode) {
       Seg seg = SegOfGreyRing(node);
-      if(TraceSetInter(SegGrey(seg), arena->flippedTraces) == TraceSetEMPTY
-          && TraceSetIsMember(SegGrey(seg), trace))
-        ShieldRaise(arena, seg, AccessREAD);
+      SegFlip(seg, trace);
     }
 
   /* @@@@ When write barrier collection is implemented, this is where */
diff --git a/mps/design/seg.txt b/mps/design/seg.txt
index 70c62a2c7dd..e4f64404dbc 100644
--- a/mps/design/seg.txt
+++ b/mps/design/seg.txt
@@ -233,6 +233,16 @@ of ``segLo`` and ``segHi``.
 Extensibility
 -------------
 
+Garbage collection
+..................
+
+``typedef void (*SegFlipMethod)(Seg seg, Trace trace)``
+
+_`.method.flip`: Raise the read barrier, if necessary, for a trace
+that's about to flip and for which the segment is grey and potentially
+contains references.
+
+
 Splitting and merging
 .....................