diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index 187046931..8551e462c 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,13 @@
+2011-05-16  Corinna Vinschen  <corinna@vinschen.de>
+
+	* dcrt0.cc (child_info_fork::alloc_stack_hard_way): Check if the
+	requested stack is application-provided within the user heap or an
+	mmapped region.  If so, just use it.  Add comment to explain why.
+	* miscfuncs.cc (thread_wrapper): If an application-provided stack
+	has been given, implement cygtls area at the stackbase.  Fix comment.
+	* mmap.cc (is_mmapped_region): New function.
+	* winsup.h (is_mmapped_region): Declare.
+
 2011-05-15  Corinna Vinschen  <corinna@vinschen.de>
 
 	* miscfuncs.cc (thread_wrapper): Add comments to assembler code.
diff --git a/winsup/cygwin/dcrt0.cc b/winsup/cygwin/dcrt0.cc
index 92056c80f..a6eb2696d 100644
--- a/winsup/cygwin/dcrt0.cc
+++ b/winsup/cygwin/dcrt0.cc
@@ -398,6 +398,15 @@ child_info_fork::alloc_stack_hard_way (volatile char *b)
   int newlen;
   bool guard;
 
+  /* First check if the requested stack area is part of the user heap
+     or part of a mmaped region.  If so, we have been started from a
+     pthread with an application-provided stack, and the stack has just
+     to be used as is. */
+  if ((stacktop >= cygheap->user_heap.base
+      && stackbottom <= cygheap->user_heap.max)
+      || is_mmapped_region ((caddr_t) stacktop, (caddr_t) stackbottom))
+    return;
+
   if (!VirtualQuery ((LPCVOID) &b, &m, sizeof m))
     api_fatal ("fork: couldn't get stack info, %E");
 
diff --git a/winsup/cygwin/miscfuncs.cc b/winsup/cygwin/miscfuncs.cc
index 73dec4c1a..a166ca1da 100644
--- a/winsup/cygwin/miscfuncs.cc
+++ b/winsup/cygwin/miscfuncs.cc
@@ -410,16 +410,17 @@ thread_wrapper (VOID *arg)
     {
       /* If the application provided the stack, we must make sure that
 	 it's actually used by the thread function.  So what we do here is
-	 to compute the stackbase of the application-provided stack and
-	 change the stack pointer accordingly.
-
-	 NOTE: _my_tls is on the stack created by CreateThread!  It's
-	       unlikely the tls structure will ever exceed 64K, but if
-	       so, we have to raise the size of the stack in the call
-	       to CreateThread, too. */
+	 to compute the stackbase of the application-provided stack, move
+	 _my_tls to the stackbase, and change the stack pointer accordingly. */
       wrapper_arg.stackaddr = (PVOID) ((PBYTE) wrapper_arg.stackaddr
-					       + wrapper_arg.stacksize
-					       - sizeof (PVOID));
+					       + wrapper_arg.stacksize);
+      _tlsbase = (char *) wrapper_arg.stackaddr;
+      wrapper_arg.stackaddr = (PVOID) ((PBYTE) wrapper_arg.stackaddr
+					       - CYGTLS_PADSIZE);
+      _tlstop = (char *) wrapper_arg.stackaddr;
+      _my_tls.init_thread ((char *) wrapper_arg.stackaddr,
+			   (DWORD (*)(void*, void*)) wrapper_arg.func);
+      wrapper_arg.stackaddr = (PVOID) (_tlstop - sizeof (PVOID));
       __asm__ ("\n\
 	       movl  %[WRAPPER_ARG], %%ebx # Load &wrapper_arg into ebx  \n\
 	       movl  (%%ebx), %%eax        # Load thread func into eax   \n\
diff --git a/winsup/cygwin/mmap.cc b/winsup/cygwin/mmap.cc
index 059daa186..9daa32fd7 100644
--- a/winsup/cygwin/mmap.cc
+++ b/winsup/cygwin/mmap.cc
@@ -659,6 +659,34 @@ mmap_areas::del_list (mmap_list *ml)
   cfree (ml);
 }
 
+/* This function allows an external function to test if a given memory
+   region is part of an mmapped memory region. */
+bool
+is_mmapped_region (caddr_t start_addr, caddr_t end_address)
+{
+  bool ret = false;
+
+  size_t len = end_address - start_addr;
+
+  LIST_LOCK ();
+  mmap_list *map_list = mmapped_areas.get_list_by_fd (-1, NULL);
+
+  mmap_record *rec;
+  caddr_t u_addr;
+  DWORD u_len;
+
+  LIST_FOREACH (rec, &map_list->recs, mr_next)
+    {
+      if (rec->match (start_addr, len, u_addr, u_len))
+	{
+	  ret = true;
+	  break;
+	}
+    }
+  LIST_UNLOCK ();
+  return ret;
+}
+
 /* This function is called from exception_handler when a segmentation
    violation has occurred.  It should also be called from all Cygwin
    functions that want to support passing noreserve mmap page addresses
@@ -677,6 +705,7 @@ mmap_areas::del_list (mmap_list *ml)
    On MAP_NORESERVE_COMMITED, the exeception handler should return 0 to
    allow the application to retry the memory access, or the calling Cygwin
    function should retry the Windows system call. */
+
 mmap_region_status
 mmap_is_attached_or_noreserve (void *addr, size_t len)
 {
diff --git a/winsup/cygwin/winsup.h b/winsup/cygwin/winsup.h
index 087da61f4..44f00339b 100644
--- a/winsup/cygwin/winsup.h
+++ b/winsup/cygwin/winsup.h
@@ -270,6 +270,7 @@ enum mmap_region_status
     MMAP_NORESERVE_COMMITED
   };
 mmap_region_status mmap_is_attached_or_noreserve (void *addr, size_t len);
+bool is_mmapped_region (caddr_t start_addr, caddr_t end_address);
 
 inline bool flush_file_buffers (HANDLE h)
 {