diff --git a/go/store/nbs/store.go b/go/store/nbs/store.go
index 266b8f792b..a0e964353f 100644
--- a/go/store/nbs/store.go
+++ b/go/store/nbs/store.go
@@ -1406,14 +1406,25 @@ func (nbs *NomsBlockStore) MarkAndSweepChunks(ctx context.Context, last hash.Has
 		return chunks.ErrUnsupportedOperation
 	}
 
-	if nbs.upstream.root != last {
-		return errLastRootMismatch
-	}
+	precheck := func() error {
+		nbs.mu.RLock()
+		defer nbs.mu.RUnlock()
 
-	// check to see if the specs have changed since last gc.  If they haven't bail early.
-	gcGenCheck := generateLockHash(last, nbs.upstream.specs, nbs.upstream.appendix)
-	if nbs.upstream.gcGen == gcGenCheck {
-		return chunks.ErrNothingToCollect
+		if nbs.upstream.root != last {
+			return errLastRootMismatch
+		}
+
+		// check to see if the specs have changed since last gc.  If they haven't bail early.
+		gcGenCheck := generateLockHash(last, nbs.upstream.specs, nbs.upstream.appendix)
+		if nbs.upstream.gcGen == gcGenCheck {
+			return chunks.ErrNothingToCollect
+		}
+
+		return nil
+	}
+	err := precheck()
+	if err != nil {
+		return err
 	}
 
 	specs, err := nbs.copyMarkedChunks(ctx, keepChunks)
@@ -1432,18 +1443,13 @@ func (nbs *NomsBlockStore) MarkAndSweepChunks(ctx context.Context, last hash.Has
 		return ctx.Err()
 	}
 
-	ok, contents, err := nbs.mm.Fetch(ctx, &Stats{})
-	if err != nil {
-		return err
-	}
-	if !ok {
-		panic("no manifest")
-	}
-	if ctx.Err() != nil {
-		return ctx.Err()
-	}
+	currentContents := func() manifestContents {
+		nbs.mu.RLock()
+		defer nbs.mu.RUnlock()
+		return nbs.upstream
+	}()
 
-	return nbs.p.PruneTableFiles(ctx, contents)
+	return nbs.p.PruneTableFiles(ctx, currentContents)
 }
 
 func (nbs *NomsBlockStore) copyMarkedChunks(ctx context.Context, keepChunks <-chan []hash.Hash) ([]tableSpec, error) {
@@ -1503,7 +1509,18 @@ func (nbs *NomsBlockStore) gcTableSize() (uint64, error) {
 	return nbs.mtSize, nil
 }
 
-func (nbs *NomsBlockStore) swapTables(ctx context.Context, specs []tableSpec) error {
+func (nbs *NomsBlockStore) swapTables(ctx context.Context, specs []tableSpec) (err error) {
+	nbs.mm.LockForUpdate()
+	defer func() {
+		unlockErr := nbs.mm.UnlockForUpdate()
+		if err == nil {
+			err = unlockErr
+		}
+	}()
+
+	nbs.mu.Lock()
+	defer nbs.mu.Unlock()
+
 	newLock := generateLockHash(nbs.upstream.root, specs, []tableSpec{})
 	newContents := manifestContents{
 		vers:  nbs.upstream.vers,
@@ -1513,19 +1530,13 @@ func (nbs *NomsBlockStore) swapTables(ctx context.Context, specs []tableSpec) er
 		specs: specs,
 	}
 
-	var err error
-	nbs.mm.LockForUpdate()
-	defer func() {
-		unlockErr := nbs.mm.UnlockForUpdate()
+	upstream, uerr := nbs.mm.UpdateGCGen(ctx, nbs.upstream.lock, newContents, nbs.stats, nil)
+	if uerr != nil {
+		return uerr
+	}
 
-		if err == nil {
-			err = unlockErr
-		}
-	}()
-
-	upstream, err := nbs.mm.UpdateGCGen(ctx, nbs.upstream.lock, newContents, nbs.stats, nil)
-	if err != nil {
-		return err
+	if upstream.lock != newContents.lock {
+		return errors.New("concurrent manifest edit during GC, before swapTables. GC failed.")
 	}
 
 	// clear memTable
@@ -1533,15 +1544,13 @@ func (nbs *NomsBlockStore) swapTables(ctx context.Context, specs []tableSpec) er
 
 	// clear nbs.tables.novel
 	nbs.tables, err = nbs.tables.Flatten()
-
 	if err != nil {
-		return nil
+		return err
 	}
 
 	// replace nbs.tables.upstream with gc compacted tables
 	nbs.upstream = upstream
-	nbs.tables, err = nbs.tables.Rebase(ctx, specs, nbs.stats)
-
+	nbs.tables, err = nbs.tables.Rebase(ctx, upstream.specs, nbs.stats)
 	if err != nil {
 		return err
 	}