Use atomics for async interrupts

* libguile/__scm.h (SCM_TICK): Always define as scm_async_tick().
* libguile/error.c (scm_syserror, scm_syserror_msg):
* libguile/fports.c (fport_read, fport_write):
* libguile/_scm.h (SCM_SYSCALL): Replace SCM_ASYNC_TICK with
  scm_async_tick ().
  (SCM_ASYNC_TICK, SCM_ASYNC_TICK_WITH_CODE)
  (SCM_ASYNC_TICK_WITH_GUARD_CODE): Remove internal definitions.  We
  inline into vm-engine.c, the only place where it matters.
* libguile/async.h:
* libguile/async.c (scm_async_tick, scm_i_setup_sleep):
  (scm_i_reset_sleep, scm_system_async_mark_for_thread):
* libguile/threads.h (struct scm_thread_wake_data):
* libguile/threads.h (scm_i_thread):
* libguile/threads.c (block_self, guilify_self_1, scm_std_select):
  Rewrite to use sequentially-consistent atomic references.
* libguile/atomics-internal.h (scm_atomic_set_pointer):
  (scm_atomic_ref_pointer): New definitions.
* libguile/finalizers.c (queue_finalizer_async): We can allocate, so
  just use scm_system_async_mark_for_thread instead of the set-cdr!
  shenanigans.
* libguile/scmsigs.c (take_signal):
* libguile/gc.c (queue_after_gc_hook): Adapt to new asyncs mechanism.
  Can't allocate but we're just manipulating the current thread when no
  other threads are running so we should be good.
* libguile/vm-engine.c (VM_HANDLE_INTERRUPTS): Inline the async_tick
  business.

14 files changed, 152 insertions, 192 deletions

diff --git a/libguile/__scm.h b/libguile/__scm.h
index 31e395285..1ea4822a6 100644
--- a/libguile/__scm.h
+++ b/libguile/__scm.h
@@ -474,11 +474,7 @@ typedef long SCM_STACKITEM;
 #define SCM_STACK_PTR(ptr) ((SCM_STACKITEM *) (void *) (ptr))
 
 
-#ifdef BUILDING_LIBGUILE
-#define SCM_TICK SCM_ASYNC_TICK
-#else
 #define SCM_TICK scm_async_tick ()
-#endif
 
 
 
diff --git a/libguile/_scm.h b/libguile/_scm.h
index 60ad08295..e482b7e31 100644
--- a/libguile/_scm.h
+++ b/libguile/_scm.h
@@ -100,7 +100,7 @@
       errno = 0;							\
       line;								\
       if (EVMSERR == errno && (vaxc$errno>>3)==(SS$_CONTROLC>>3))	\
-	SCM_ASYNC_TICK;							\
+        scm_async_tick ();                                              \
       else								\
 	break;								\
     }									\
@@ -119,7 +119,7 @@
       line;					\
       if (errno == EINTR)			\
 	{					\
-	  SCM_ASYNC_TICK;			\
+          scm_async_tick ();                    \
 	  errno = EINTR;			\
 	}					\
     }						\
@@ -225,26 +225,6 @@ void scm_ia64_longjmp (scm_i_jmp_buf *, int);
 
 
 
-#define SCM_ASYNC_TICK_WITH_GUARD_CODE(thr, pre, post)                  \
-  do                                                                    \
-    {                                                                   \
-      if (SCM_UNLIKELY (thr->pending_asyncs))                           \
-        {                                                               \
-          pre;                                                          \
-          scm_async_tick ();                                            \
-          post;                                                         \
-        }                                                               \
-    }                                                                   \
-  while (0)
-
-#define SCM_ASYNC_TICK_WITH_CODE(thr, stmt) \
-  SCM_ASYNC_TICK_WITH_GUARD_CODE (thr, stmt, (void) 0)
-#define SCM_ASYNC_TICK \
-  SCM_ASYNC_TICK_WITH_CODE (SCM_I_CURRENT_THREAD, (void) 0)
-
-
-
-
 #if (defined __GNUC__)
 # define SCM_NOINLINE __attribute__ ((__noinline__))
 #else
diff --git a/libguile/async.c b/libguile/async.c
index 1cf105881..aa4f508b7 100644
--- a/libguile/async.c
+++ b/libguile/async.c
@@ -24,6 +24,7 @@
 #endif
 
 #include "libguile/_scm.h"
+#include "libguile/atomics-internal.h"
 #include "libguile/eval.h"
 #include "libguile/throw.h"
 #include "libguile/root.h"
@@ -50,142 +51,51 @@
  *
  * Each thread has a list of 'activated asyncs', which is a normal
  * Scheme list of procedures with zero arguments.  When a thread
- * executes a SCM_ASYNC_TICK statement (which is included in SCM_TICK),
- * it will call all procedures on this list.
+ * executes an scm_async_tick (), it will call all procedures on this
+ * list.
  */
 
 
-
-
-static scm_i_pthread_mutex_t async_mutex = SCM_I_PTHREAD_MUTEX_INITIALIZER;
-
-/* System asyncs. */
-
 void
 scm_async_tick (void)
 {
   scm_i_thread *t = SCM_I_CURRENT_THREAD;
   SCM asyncs;
 
-  /* Reset pending_asyncs even when asyncs are blocked and not really
-     executed since this will avoid future futile calls to this
-     function.  When asyncs are unblocked again, this function is
-     invoked even when pending_asyncs is zero.
-  */
-
-  scm_i_scm_pthread_mutex_lock (&async_mutex);
-  t->pending_asyncs = 0;
-  if (t->block_asyncs == 0)
-    {
-      asyncs = t->active_asyncs;
-      t->active_asyncs = SCM_EOL;
-    }
-  else
-    asyncs = SCM_EOL;
-  scm_i_pthread_mutex_unlock (&async_mutex);
+  if (t->block_asyncs)
+    return;
 
-  while (scm_is_pair (asyncs))
+  asyncs = scm_atomic_swap_scm (&t->pending_asyncs, SCM_EOL);
+  while (!scm_is_null (asyncs))
     {
-      SCM next = SCM_CDR (asyncs);
-      SCM_SETCDR (asyncs, SCM_BOOL_F);
-      scm_call_0 (SCM_CAR (asyncs));
+      SCM next = scm_cdr (asyncs);
+      scm_call_0 (scm_car (asyncs));
+      scm_set_cdr_x (asyncs, SCM_BOOL_F);
       asyncs = next;
     }
 }
 
-void
-scm_i_queue_async_cell (SCM c, scm_i_thread *t)
-{
-  SCM sleep_object;
-  scm_i_pthread_mutex_t *sleep_mutex;
-  int sleep_fd;
-  SCM p;
-  
-  scm_i_scm_pthread_mutex_lock (&async_mutex);
-  p = t->active_asyncs;
-  SCM_SETCDR (c, SCM_EOL);
-  if (!scm_is_pair (p))
-    t->active_asyncs = c;
-  else
-    {
-      SCM pp;
-      while (scm_is_pair (pp = SCM_CDR (p)))
-	{
-	  if (scm_is_eq (SCM_CAR (p), SCM_CAR (c)))
-	    {
-	      scm_i_pthread_mutex_unlock (&async_mutex);
-	      return;
-	    }
-	  p = pp;
-	}
-      SCM_SETCDR (p, c);
-    }
-  t->pending_asyncs = 1;
-  sleep_object = t->sleep_object;
-  sleep_mutex = t->sleep_mutex;
-  sleep_fd = t->sleep_fd;
-  scm_i_pthread_mutex_unlock (&async_mutex);
-
-  if (sleep_mutex)
-    {
-      /* By now, the thread T might be out of its sleep already, or
-	 might even be in the next, unrelated sleep.  Interrupting it
-	 anyway does no harm, however.
-
-	 The important thing to prevent here is to signal sleep_cond
-	 before T waits on it.  This can not happen since T has
-	 sleep_mutex locked while setting t->sleep_mutex and will only
-	 unlock it again while waiting on sleep_cond.
-      */
-      scm_i_scm_pthread_mutex_lock (sleep_mutex);
-      scm_i_pthread_cond_signal (&t->sleep_cond);
-      scm_i_pthread_mutex_unlock (sleep_mutex);
-    }
-
-  if (sleep_fd >= 0)
-    {
-      char dummy = 0;
-
-      /* Likewise, T might already been done with sleeping here, but
-	 interrupting it once too often does no harm.  T might also
-	 not yet have started sleeping, but this is no problem either
-	 since the data written to a pipe will not be lost, unlike a
-	 condition variable signal.  */
-      full_write (sleep_fd, &dummy, 1);
-    }
-
-  /* This is needed to protect sleep_mutex.
-   */
-  scm_remember_upto_here_1 (sleep_object);
-}
-
 int
 scm_i_setup_sleep (scm_i_thread *t,
 		   SCM sleep_object, scm_i_pthread_mutex_t *sleep_mutex,
 		   int sleep_fd)
 {
-  int pending;
+  struct scm_thread_wake_data *wake;
 
-  scm_i_scm_pthread_mutex_lock (&async_mutex);
-  pending = t->pending_asyncs;
-  if (!pending)
-    {
-      t->sleep_object = sleep_object;
-      t->sleep_mutex = sleep_mutex;
-      t->sleep_fd = sleep_fd;
-    }
-  scm_i_pthread_mutex_unlock (&async_mutex);
-  return pending;
+  wake = scm_gc_typed_calloc (struct scm_thread_wake_data);
+  wake->object = sleep_object;
+  wake->mutex = sleep_mutex;
+  wake->fd = sleep_fd;
+
+  scm_atomic_set_pointer ((void **)&t->wake, wake);
+
+  return !scm_is_null (scm_atomic_ref_scm (&t->pending_asyncs));
 }
 
 void
 scm_i_reset_sleep (scm_i_thread *t)
 {
-  scm_i_scm_pthread_mutex_lock (&async_mutex);
-  t->sleep_object = SCM_BOOL_F;
-  t->sleep_mutex = NULL;
-  t->sleep_fd = -1;
-  scm_i_pthread_mutex_unlock (&async_mutex);  
+  scm_atomic_set_pointer ((void **)&t->wake, NULL);
 }
 
 SCM_DEFINE (scm_system_async_mark_for_thread, "system-async-mark", 1, 1, 0,
@@ -200,13 +110,9 @@ SCM_DEFINE (scm_system_async_mark_for_thread, "system-async-mark", 1, 1, 0,
 	    "signal handlers.")
 #define FUNC_NAME s_scm_system_async_mark_for_thread
 {
-  /* The current thread might not have a handle yet.  This can happen
-     when the GC runs immediately before allocating the handle.  At
-     the end of that GC, a system async might be marked.  Thus, we can
-     not use scm_current_thread here.
-  */
-
   scm_i_thread *t;
+  SCM asyncs;
+  struct scm_thread_wake_data *wake;
 
   if (SCM_UNBNDP (thread))
     t = SCM_I_CURRENT_THREAD;
@@ -217,7 +123,48 @@ SCM_DEFINE (scm_system_async_mark_for_thread, "system-async-mark", 1, 1, 0,
 	SCM_MISC_ERROR ("thread has already exited", SCM_EOL);
       t = SCM_I_THREAD_DATA (thread);
     }
-  scm_i_queue_async_cell (scm_cons (proc, SCM_BOOL_F), t);
+
+  asyncs = scm_atomic_ref_scm (&t->pending_asyncs);
+  do
+    if (scm_is_true (scm_c_memq (proc, asyncs)))
+      return SCM_UNSPECIFIED;
+  while (!scm_atomic_compare_and_swap_scm (&t->pending_asyncs, &asyncs,
+                                           scm_cons (proc, asyncs)));
+
+  /* At this point the async is enqueued.  However if the thread is
+     sleeping, we have to wake it up.  */
+  if ((wake = scm_atomic_ref_pointer ((void **) &t->wake)))
+    {
+      /* By now, the thread T might be out of its sleep already, or
+	 might even be in the next, unrelated sleep.  Interrupting it
+	 anyway does no harm, however.
+
+	 The important thing to prevent here is to signal sleep_cond
+	 before T waits on it.  This can not happen since T has
+	 sleep_mutex locked while setting t->sleep_mutex and will only
+	 unlock it again while waiting on sleep_cond.
+      */
+      scm_i_scm_pthread_mutex_lock (wake->mutex);
+      scm_i_pthread_cond_signal (&t->sleep_cond);
+      scm_i_pthread_mutex_unlock (wake->mutex);
+
+      /* This is needed to protect wake->mutex.
+       */
+      scm_remember_upto_here_1 (wake->object);
+
+      if (wake->fd >= 0)
+        {
+          char dummy = 0;
+
+          /* Likewise, T might already been done with sleeping here, but
+             interrupting it once too often does no harm.  T might also
+             not yet have started sleeping, but this is no problem
+             either since the data written to a pipe will not be lost,
+             unlike a condition variable signal.  */
+          full_write (wake->fd, &dummy, 1);
+        }
+    }
+
   return SCM_UNSPECIFIED;
 }
 #undef FUNC_NAME
diff --git a/libguile/async.h b/libguile/async.h
index 1e9760a58..e8b6ee9c2 100644
--- a/libguile/async.h
+++ b/libguile/async.h
@@ -34,7 +34,6 @@ SCM_API void scm_async_tick (void);
 SCM_API void scm_switch (void);
 SCM_API SCM scm_system_async_mark (SCM a);
 SCM_API SCM scm_system_async_mark_for_thread (SCM a, SCM thread);
-SCM_INTERNAL void scm_i_queue_async_cell (SCM cell, scm_i_thread *);
 SCM_INTERNAL int scm_i_setup_sleep (scm_i_thread *,
 				    SCM obj, scm_i_pthread_mutex_t *m,
 				    int fd);
diff --git a/libguile/atomics-internal.h b/libguile/atomics-internal.h
index 9d18cbc1a..9074d8cc3 100644
--- a/libguile/atomics-internal.h
+++ b/libguile/atomics-internal.h
@@ -45,6 +45,16 @@ scm_atomic_compare_and_swap_uint32 (uint32_t *loc, uint32_t *expected,
   return atomic_compare_exchange_weak (loc, expected, desired);
 }
 static inline void
+scm_atomic_set_pointer (void **loc, void *val)
+{
+  atomic_store (loc, val);
+}
+static inline void *
+scm_atomic_ref_pointer (void **loc)
+{
+  return atomic_load (loc);
+}
+static inline void
 scm_atomic_set_scm (SCM *loc, SCM val)
 {
   atomic_store (loc, val);
@@ -100,6 +110,23 @@ scm_atomic_compare_and_swap_uint32 (uint32_t *loc, uint32_t *expected,
 }
 
 static inline void
+scm_atomic_set_pointer (void **loc, void *val)
+{
+  scm_i_pthread_mutex_lock (&atomics_lock);
+  *loc = val;
+  scm_i_pthread_mutex_unlock (&atomics_lock);
+}
+static inline void *
+scm_atomic_ref_pointer (void **loc)
+{
+  void *ret;
+  scm_i_pthread_mutex_lock (&atomics_lock);
+  ret = *loc;
+  scm_i_pthread_mutex_unlock (&atomics_lock);
+  return ret;
+}
+
+static inline void
 scm_atomic_set_scm (SCM *loc, SCM val)
 {
   scm_i_pthread_mutex_lock (&atomics_lock);
diff --git a/libguile/error.c b/libguile/error.c
index 89345c2b7..ff84f41d8 100644
--- a/libguile/error.c
+++ b/libguile/error.c
@@ -163,7 +163,7 @@ scm_syserror (const char *subr)
   */
 #ifdef EINTR
   if (scm_to_int (err) == EINTR)
-    SCM_ASYNC_TICK;
+    scm_async_tick ();
 #endif
 
   scm_error (scm_system_error_key,
@@ -179,7 +179,7 @@ scm_syserror_msg (const char *subr, const char *message, SCM args, int eno)
   /* See above note about the EINTR signal handling race. */
 #ifdef EINTR
   if (eno == EINTR)
-    SCM_ASYNC_TICK;
+    scm_async_tick ();
 #endif
   scm_error (scm_system_error_key,
 	     subr,
diff --git a/libguile/finalizers.c b/libguile/finalizers.c
index 82f292cd2..9b9075830 100644
--- a/libguile/finalizers.c
+++ b/libguile/finalizers.c
@@ -40,6 +40,8 @@ static int automatic_finalization_p = 1;
 
 static size_t finalization_count;
 
+static SCM run_finalizers_subr;
+
 
 
 
@@ -132,8 +134,6 @@ scm_i_add_finalizer (void *obj, scm_t_finalizer_proc proc, void *data)
 
 
 
-static SCM finalizer_async_cell;
-
 static SCM
 run_finalizers_async_thunk (void)
 {
@@ -150,19 +150,13 @@ static void
 queue_finalizer_async (void)
 {
   scm_i_thread *t = SCM_I_CURRENT_THREAD;
-  static scm_i_pthread_mutex_t lock = SCM_I_PTHREAD_MUTEX_INITIALIZER;
 
-  scm_i_pthread_mutex_lock (&lock);
-  /* If t is NULL, that could be because we're allocating in
-     threads.c:guilify_self_1.  In that case, rely on the
+  /* Could be that the current thread is is NULL when we're allocating
+     in threads.c:guilify_self_1.  In that case, rely on the
      GC_invoke_finalizers call there after the thread spins up.  */
-  if (t && scm_is_false (SCM_CDR (finalizer_async_cell)))
-    {
-      SCM_SETCDR (finalizer_async_cell, t->active_asyncs);
-      t->active_asyncs = finalizer_async_cell;
-      t->pending_asyncs = 1;
-    }
-  scm_i_pthread_mutex_unlock (&lock);
+  if (!t) return;
+
+  scm_system_async_mark_for_thread (run_finalizers_subr, t->handle);
 }
 
 
@@ -418,10 +412,8 @@ scm_init_finalizers (void)
 {
   /* When the async is to run, the cdr of the pair gets set to the
      asyncs queue of the current thread.  */
-  finalizer_async_cell =
-    scm_cons (scm_c_make_gsubr ("%run-finalizers", 0, 0, 0,
-                                run_finalizers_async_thunk),
-              SCM_BOOL_F);
+  run_finalizers_subr = scm_c_make_gsubr ("%run-finalizers", 0, 0, 0,
+                                          run_finalizers_async_thunk);
 
   if (automatic_finalization_p)
     GC_set_finalizer_notifier (queue_finalizer_async);
diff --git a/libguile/fports.c b/libguile/fports.c
index 5886f628d..8fa69933d 100644
--- a/libguile/fports.c
+++ b/libguile/fports.c
@@ -595,7 +595,7 @@ fport_read (SCM port, SCM dst, size_t start, size_t count)
     {
       if (errno == EINTR)
         {
-          SCM_ASYNC_TICK;
+          scm_async_tick ();
           goto retry;
         }
       if (errno == EWOULDBLOCK || errno == EAGAIN)
@@ -618,7 +618,7 @@ fport_write (SCM port, SCM src, size_t start, size_t count)
     {
       if (errno == EINTR)
         {
-          SCM_ASYNC_TICK;
+          scm_async_tick ();
           goto retry;
         }
       if (errno == EWOULDBLOCK || errno == EAGAIN)
diff --git a/libguile/gc.c b/libguile/gc.c
index b75a688aa..1e9f59683 100644
--- a/libguile/gc.c
+++ b/libguile/gc.c
@@ -685,8 +685,8 @@ after_gc_async_thunk (void)
  */
 static void *
 queue_after_gc_hook (void * hook_data SCM_UNUSED,
-                      void *fn_data SCM_UNUSED,
-                      void *data SCM_UNUSED)
+                     void *fn_data SCM_UNUSED,
+                     void *data SCM_UNUSED)
 {
   /* If cell access debugging is enabled, the user may choose to perform
    * additional garbage collections after an arbitrary number of cell
@@ -721,9 +721,8 @@ queue_after_gc_hook (void * hook_data SCM_UNUSED,
 
       if (scm_is_false (SCM_CDR (after_gc_async_cell)))
         {
-          SCM_SETCDR (after_gc_async_cell, t->active_asyncs);
-          t->active_asyncs = after_gc_async_cell;
-          t->pending_asyncs = 1;
+          SCM_SETCDR (after_gc_async_cell, t->pending_asyncs);
+          t->pending_asyncs = after_gc_async_cell;
         }
     }
 
diff --git a/libguile/init.c b/libguile/init.c
index 4b95f3612..8b0813a1b 100644
--- a/libguile/init.c
+++ b/libguile/init.c
@@ -344,7 +344,7 @@ invoke_main_func (void *body_data)
    * asyncs a chance to run.  This must be done after
    * the call to scm_restore_signals.
    */
-  SCM_ASYNC_TICK;
+  scm_async_tick ();
 
   /* Indicate success by returning non-NULL.
    */
diff --git a/libguile/scmsigs.c b/libguile/scmsigs.c
index 441da3ec7..b030b0413 100644
--- a/libguile/scmsigs.c
+++ b/libguile/scmsigs.c
@@ -228,9 +228,8 @@ take_signal (int signum)
 
   if (scm_is_false (SCM_CDR (cell)))
     {
-      SCM_SETCDR (cell, t->active_asyncs);
-      t->active_asyncs = cell;
-      t->pending_asyncs = 1;
+      SCM_SETCDR (cell, t->pending_asyncs);
+      t->pending_asyncs = cell;
     }
 
 #ifndef HAVE_SIGACTION
diff --git a/libguile/threads.c b/libguile/threads.c
index 1dece5694..2a315e472 100644
--- a/libguile/threads.c
+++ b/libguile/threads.c
@@ -275,7 +275,7 @@ thread_print (SCM exp, SCM port, scm_print_state *pstate SCM_UNUSED)
 
 /*** Blocking on queues. */
 
-/* See also scm_i_queue_async_cell for how such a block is
+/* See also scm_system_async_mark_for_thread for how such a block is
    interrputed.
 */
 
@@ -309,7 +309,10 @@ block_self (SCM queue, SCM sleep_object, scm_i_pthread_mutex_t *mutex,
   int err;
 
   if (scm_i_setup_sleep (t, sleep_object, mutex, -1))
-    err = EINTR;
+    {
+      scm_i_reset_sleep (t);
+      err = EINTR;
+    }
   else
     {
       t->block_asyncs++;
@@ -415,9 +418,8 @@ guilify_self_1 (struct GC_stack_base *base)
   t.dynstack.base = NULL;
   t.dynstack.top = NULL;
   t.dynstack.limit = NULL;
-  t.active_asyncs = SCM_EOL;
+  t.pending_asyncs = SCM_EOL;
   t.block_asyncs = 1;
-  t.pending_asyncs = 1;
   t.critical_section_level = 0;
   t.base = base->mem_base;
 #ifdef __ia64__
@@ -426,9 +428,7 @@ guilify_self_1 (struct GC_stack_base *base)
   t.continuation_root = SCM_EOL;
   t.continuation_base = t.base;
   scm_i_pthread_cond_init (&t.sleep_cond, NULL);
-  t.sleep_mutex = NULL;
-  t.sleep_object = SCM_BOOL_F;
-  t.sleep_fd = -1;
+  t.wake = NULL;
   t.vp = NULL;
 
   if (pipe2 (t.sleep_pipe, O_CLOEXEC) != 0)
@@ -1776,7 +1776,10 @@ scm_std_select (int nfds,
     }
 
   while (scm_i_setup_sleep (t, SCM_BOOL_F, NULL, t->sleep_pipe[1]))
-    SCM_TICK;
+    {
+      scm_i_reset_sleep (t);
+      SCM_TICK;
+    }
 
   wakeup_fd = t->sleep_pipe[0];
   FD_SET (wakeup_fd, readfds);
@@ -1795,7 +1798,6 @@ scm_std_select (int nfds,
   res = args.result;
   eno = args.errno_value;
 
-  t->sleep_fd = -1;
   scm_i_reset_sleep (t);
 
   if (res > 0 && FD_ISSET (wakeup_fd, readfds))
diff --git a/libguile/threads.h b/libguile/threads.h
index a8bb21a4a..241907db2 100644
--- a/libguile/threads.h
+++ b/libguile/threads.h
@@ -47,6 +47,13 @@ SCM_API scm_t_bits scm_tc16_thread;
 SCM_API scm_t_bits scm_tc16_mutex;
 SCM_API scm_t_bits scm_tc16_condvar;
 
+struct scm_thread_wake_data
+{
+  SCM object;
+  scm_i_pthread_mutex_t *mutex;
+  int fd;
+};
+
 typedef struct scm_i_thread {
   struct scm_i_thread *next_thread;
 
@@ -67,10 +74,9 @@ typedef struct scm_i_thread {
   /* Boolean indicating whether the thread is in guile mode.  */
   int guile_mode;
 
-  SCM sleep_object;
-  scm_i_pthread_mutex_t *sleep_mutex;
+  struct scm_thread_wake_data *wake;
   scm_i_pthread_cond_t sleep_cond;
-  int sleep_fd, sleep_pipe[2];
+  int sleep_pipe[2];
 
   /* Thread-local freelists; see gc-inline.h.  */
   void **freelists;
@@ -85,12 +91,10 @@ typedef struct scm_i_thread {
 
   /* For system asyncs.
    */
-  SCM active_asyncs;            /* The thunks to be run at the next
-                                   safe point */
+  SCM pending_asyncs;            /* The thunks to be run at the next
+                                    safe point.  Accessed atomically. */
   unsigned int block_asyncs;    /* Non-zero means that asyncs should
                                    not be run. */
-  unsigned int pending_asyncs;  /* Non-zero means that asyncs might be pending.
-				 */
 
   /* The current continuation root and the stack base for it.
 
diff --git a/libguile/vm-engine.c b/libguile/vm-engine.c
index 852e10d06..4f66b9e7d 100644
--- a/libguile/vm-engine.c
+++ b/libguile/vm-engine.c
@@ -127,9 +127,24 @@
 #define ABORT_CONTINUATION_HOOK()               \
   RUN_HOOK0 (abort)
 
-#define VM_HANDLE_INTERRUPTS                     \
-  SCM_ASYNC_TICK_WITH_GUARD_CODE (thread, SYNC_IP (), CACHE_SP ())
+/* TODO: Invoke asyncs without trampolining out to C.  That will let us
+   preempt computations via an asynchronous interrupt.  */
+#define VM_HANDLE_INTERRUPTS                                            \
+  do                                                                    \
+    if (SCM_LIKELY (thread->block_asyncs == 0))                         \
+      {                                                                 \
+        SCM asyncs = scm_atomic_ref_scm (&thread->pending_asyncs);      \
+        if (SCM_UNLIKELY (!scm_is_null (asyncs)))                       \
+          {                                                             \
+            SYNC_IP ();                                                 \
+            scm_async_tick ();                                          \
+            CACHE_SP ();                                                \
+          }                                                             \
+      }                                                                 \
+  while (0)
+
 
+
 
 /* Virtual Machine