diff --git a/racket/src/racket/sconfig.h b/racket/src/racket/sconfig.h
index 898fab8c5a..3abd64aa20 100644
--- a/racket/src/racket/sconfig.h
+++ b/racket/src/racket/sconfig.h
@@ -322,15 +322,12 @@
 # endif
 
 # include <sys/param.h>
-# if OpenBSD < 201211
-/* This is needed for (pre-5.2) userspace threads: */
-#  define ASSUME_FIXED_STACK_SIZE
-#  define FIXED_STACK_SIZE 1048576
-# endif
 
 # include "uconfig.h"
 # undef HAS_STANDARD_IOB
 # define HAS_BSD_IOB
+# undef UNIX_FIND_STACK_BOUNDS
+# define PTHREAD_STACKSEG_FIND_STACK_BOUNDS
 
 /* Default UNIX_STACK_MAXIMUM is too big for a non-root user. */
 # undef UNIX_STACK_MAXIMUM
@@ -1515,6 +1512,8 @@
      line.
     ASSUME_FIXED_STACK_SIZE assumes that the main stack size is
      always FIXED_STACK_SIZE.
+    PTHREAD_STACKSEG_FIND_STACK_BOUNDS finds stack bounds using
+     pthread_stackseg_np().
     Use only one of these if DO_STACK_CHECK is used, or none otherwise. */
 
  /* FIXED_STACK_SIZE <X> sets the stack size to <X> when the
diff --git a/racket/src/racket/src/eval.c b/racket/src/racket/src/eval.c
index 02719e9705..2ab4aea549 100644
--- a/racket/src/racket/src/eval.c
+++ b/racket/src/racket/src/eval.c
@@ -144,6 +144,11 @@
 #include <sys/time.h>
 #include <sys/resource.h>
 #endif
+#ifdef PTHREAD_STACKSEG_FIND_STACK_BOUNDS
+# include <sys/signal.h>
+# include <pthread.h>
+# include <pthread_np.h>
+#endif
 #ifdef WINDOWS_FIND_STACK_BOUNDS
 #include <windows.h>
 #endif
@@ -595,9 +600,6 @@ void scheme_init_stack_check()
 {
   int *v, stack_grows_up;
   uintptr_t deeper;
-#ifdef UNIX_FIND_STACK_BOUNDS
-  struct rlimit rl;
-#endif
   
   deeper = scheme_get_deeper_address();
   stack_grows_up = (deeper > (uintptr_t)&v);
@@ -665,12 +667,14 @@ void scheme_init_stack_check()
 # endif
 
 # ifdef UNIX_FIND_STACK_BOUNDS
-    getrlimit(RLIMIT_STACK, &rl);
-  
     {
+      struct rlimit rl;
       uintptr_t bnd, lim;
+
       bnd = (uintptr_t)scheme_get_current_os_thread_stack_base();
 
+      getrlimit(RLIMIT_STACK, &rl);
+
 #  ifdef LINUX_FIND_STACK_BASE
       bnd = adjust_stack_base(bnd);
 #  endif
@@ -689,6 +693,14 @@ void scheme_init_stack_check()
       scheme_stack_boundary = bnd;
     }
 # endif
+
+# ifdef PTHREAD_STACKSEG_FIND_STACK_BOUNDS
+    {
+      stack_t stack;
+      pthread_stackseg_np(pthread_self(), &stack);
+      scheme_stack_boundary = (uintptr_t)((char *)stack.ss_sp - (stack.ss_size - STACK_SAFETY_MARGIN));
+    }
+# endif
   }
 #endif
 
diff --git a/racket/src/racket/src/module.c b/racket/src/racket/src/module.c
index 23376b12eb..9c27d20790 100644
--- a/racket/src/racket/src/module.c
+++ b/racket/src/racket/src/module.c
@@ -28,7 +28,7 @@
 #include "schmach.h"
 #include "schexpobs.h"
 
-#define MIN(l,o) ((l) < (o) ? (l) : (o))
+#define mz_MIN(l,o) ((l) < (o) ? (l) : (o))
 
 /* globals */
 SHARED_OK Scheme_Object *(*scheme_module_demand_hook)(int, Scheme_Object **);
@@ -3673,7 +3673,7 @@ int scheme_resolved_module_path_value_matches(Scheme_Object *rmp, Scheme_Object
   else if (SCHEME_BYTE_STRINGP(rmp_val) && SCHEME_SYMBOLP(o)) {
     return !strncmp(SCHEME_BYTE_STR_VAL(rmp_val), 
                     SCHEME_SYM_VAL(o), 
-                    MIN(SCHEME_BYTE_STRLEN_VAL(rmp_val), SCHEME_SYM_LEN(o)));
+                    mz_MIN(SCHEME_BYTE_STRLEN_VAL(rmp_val), SCHEME_SYM_LEN(o)));
   }  else {
     scheme_arg_mismatch("scheme_resolved_module_path_value_matches", 
                         "internal error: unknown type of resolved_module_path_value",
diff --git a/racket/src/racket/src/mzstkchk.h b/racket/src/racket/src/mzstkchk.h
index fbe2fc6e36..402af226a8 100644
--- a/racket/src/racket/src/mzstkchk.h
+++ b/racket/src/racket/src/mzstkchk.h
@@ -21,7 +21,7 @@
 # if defined(UNIX_FIND_STACK_BOUNDS) || defined(WINDOWS_FIND_STACK_BOUNDS) \
      || defined(MACOS_FIND_STACK_BOUNDS) || defined(ASSUME_FIXED_STACK_SIZE) \
      || defined(BEOS_FIND_STACK_BOUNDS) || defined(OSKIT_FIXED_STACK_BOUNDS) \
-     || defined(PALM_FIND_STACK_BOUNDS)
+     || defined(PALM_FIND_STACK_BOUNDS) || defined(PTHREAD_STACKSEG_FIND_STACK_BOUNDS)
   uintptr_t _stk_pos;
 
   _stk_pos = (uintptr_t)&_stk_pos;
diff --git a/racket/src/racket/src/schpriv.h b/racket/src/racket/src/schpriv.h
index d2697c94e9..798147fad6 100644
--- a/racket/src/racket/src/schpriv.h
+++ b/racket/src/racket/src/schpriv.h
@@ -1705,7 +1705,7 @@ typedef struct Scheme_Overflow {
 #if defined(UNIX_FIND_STACK_BOUNDS) || defined(WINDOWS_FIND_STACK_BOUNDS) \
     || defined(MACOS_FIND_STACK_BOUNDS) || defined(ASSUME_FIXED_STACK_SIZE) \
     || defined(BEOS_FIND_STACK_BOUNDS) || defined(OSKIT_FIXED_STACK_BOUNDS) \
-    || defined(PALM_FIND_STACK_BOUNDS)
+    || defined(PALM_FIND_STACK_BOUNDS) || defined(PTHREAD_STACKSEG_FIND_STACK_BOUNDS)
 # define USE_STACK_BOUNDARY_VAR
 THREAD_LOCAL_DECL(extern uintptr_t scheme_stack_boundary);
 /* Same as scheme_stack_boundary, but set to an extreme value when feul auto-expires,