Index: src/test_multiplex.c
==================================================================
--- src/test_multiplex.c
+++ src/test_multiplex.c
@@ -753,22 +753,26 @@
   sqlite3_int64 iOfst
 ){
   multiplexConn *p = (multiplexConn*)pConn;
   multiplexGroup *pGroup = p->pGroup;
   int rc = SQLITE_OK;
-  multiplexEnter();
+  int nMutex = 0;
+  multiplexEnter(); nMutex++;
   if( !pGroup->bEnabled ){
     sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL, 0);
+    multiplexLeave(); nMutex--;
     if( pSubOpen==0 ){
       rc = SQLITE_IOERR_READ;
     }else{
       rc = pSubOpen->pMethods->xRead(pSubOpen, pBuf, iAmt, iOfst);
     }
   }else{
     while( iAmt > 0 ){
       int i = (int)(iOfst / pGroup->szChunk);
+      if( nMutex==0 ){ multiplexEnter(); nMutex++; }
       sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, i, &rc, NULL, 1);
+      multiplexLeave(); nMutex--;
       if( pSubOpen ){
         int extra = ((int)(iOfst % pGroup->szChunk) + iAmt) - pGroup->szChunk;
         if( extra<0 ) extra = 0;
         iAmt -= extra;
         rc = pSubOpen->pMethods->xRead(pSubOpen, pBuf, iAmt,
@@ -781,11 +785,12 @@
         rc = SQLITE_IOERR_READ;
         break;
       }
     }
   }
-  multiplexLeave();
+  assert( nMutex==0 || nMutex==1 );
+  if( nMutex ) multiplexLeave();
   return rc;
 }
 
 /* Pass xWrite requests thru to the original VFS after
 ** determining the correct chunk to operate on.