diff --git a/aio-posix.c b/aio-posix.c
index 4abec38..249889f 100644
--- a/aio-posix.c
+++ b/aio-posix.c
@@ -233,26 +233,23 @@
 bool aio_poll(AioContext *ctx, bool blocking)
 {
     AioHandler *node;
-    bool was_dispatching;
     int i, ret;
     bool progress;
     int64_t timeout;
 
     aio_context_acquire(ctx);
-    was_dispatching = ctx->dispatching;
     progress = false;
 
     /* aio_notify can avoid the expensive event_notifier_set if
      * everything (file descriptors, bottom halves, timers) will
      * be re-evaluated before the next blocking poll().  This is
      * already true when aio_poll is called with blocking == false;
-     * if blocking == true, it is only true after poll() returns.
-     *
-     * If we're in a nested event loop, ctx->dispatching might be true.
-     * In that case we can restore it just before returning, but we
-     * have to clear it now.
+     * if blocking == true, it is only true after poll() returns,
+     * so disable the optimization now.
      */
-    aio_set_dispatching(ctx, !blocking);
+    if (blocking) {
+        atomic_add(&ctx->notify_me, 2);
+    }
 
     ctx->walking_handlers++;
 
@@ -272,6 +269,9 @@
         aio_context_release(ctx);
     }
     ret = qemu_poll_ns((GPollFD *)pollfds, npfd, timeout);
+    if (blocking) {
+        atomic_sub(&ctx->notify_me, 2);
+    }
     if (timeout) {
         aio_context_acquire(ctx);
     }
@@ -287,12 +287,10 @@
     ctx->walking_handlers--;
 
     /* Run dispatch even if there were no readable fds to run timers */
-    aio_set_dispatching(ctx, true);
     if (aio_dispatch(ctx)) {
         progress = true;
     }
 
-    aio_set_dispatching(ctx, was_dispatching);
     aio_context_release(ctx);
 
     return progress;
diff --git a/aio-win32.c b/aio-win32.c
index 9268b5c..ea655b0 100644
--- a/aio-win32.c
+++ b/aio-win32.c
@@ -279,25 +279,23 @@
 {
     AioHandler *node;
     HANDLE events[MAXIMUM_WAIT_OBJECTS + 1];
-    bool was_dispatching, progress, have_select_revents, first;
+    bool progress, have_select_revents, first;
     int count;
     int timeout;
 
     aio_context_acquire(ctx);
-    was_dispatching = ctx->dispatching;
     progress = false;
 
     /* aio_notify can avoid the expensive event_notifier_set if
      * everything (file descriptors, bottom halves, timers) will
      * be re-evaluated before the next blocking poll().  This is
      * already true when aio_poll is called with blocking == false;
-     * if blocking == true, it is only true after poll() returns.
-     *
-     * If we're in a nested event loop, ctx->dispatching might be true.
-     * In that case we can restore it just before returning, but we
-     * have to clear it now.
+     * if blocking == true, it is only true after poll() returns,
+     * so disable the optimization now.
      */
-    aio_set_dispatching(ctx, !blocking);
+    if (blocking) {
+        atomic_add(&ctx->notify_me, 2);
+    }
 
     have_select_revents = aio_prepare(ctx);
 
@@ -331,10 +329,13 @@
             aio_context_release(ctx);
         }
         ret = WaitForMultipleObjects(count, events, FALSE, timeout);
+        if (blocking) {
+            assert(first);
+            atomic_sub(&ctx->notify_me, 2);
+        }
         if (timeout) {
             aio_context_acquire(ctx);
         }
-        aio_set_dispatching(ctx, true);
 
         if (first && aio_bh_poll(ctx)) {
             progress = true;
@@ -358,7 +359,6 @@
 
     progress |= timerlistgroup_run_timers(&ctx->tlg);
 
-    aio_set_dispatching(ctx, was_dispatching);
     aio_context_release(ctx);
     return progress;
 }
diff --git a/async.c b/async.c
index 77d080d..a232192 100644
--- a/async.c
+++ b/async.c
@@ -184,6 +184,8 @@
 {
     AioContext *ctx = (AioContext *) source;
 
+    atomic_or(&ctx->notify_me, 1);
+
     /* We assume there is no timeout already supplied */
     *timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));
 
@@ -200,6 +202,7 @@
     AioContext *ctx = (AioContext *) source;
     QEMUBH *bh;
 
+    atomic_and(&ctx->notify_me, ~1);
     for (bh = ctx->first_bh; bh; bh = bh->next) {
         if (!bh->deleted && bh->scheduled) {
             return true;
@@ -254,23 +257,13 @@
     return ctx->thread_pool;
 }
 
-void aio_set_dispatching(AioContext *ctx, bool dispatching)
-{
-    ctx->dispatching = dispatching;
-    if (!dispatching) {
-        /* Write ctx->dispatching before reading e.g. bh->scheduled.
-         * Optimization: this is only needed when we're entering the "unsafe"
-         * phase where other threads must call event_notifier_set.
-         */
-        smp_mb();
-    }
-}
-
 void aio_notify(AioContext *ctx)
 {
-    /* Write e.g. bh->scheduled before reading ctx->dispatching.  */
+    /* Write e.g. bh->scheduled before reading ctx->notify_me.  Pairs
+     * with atomic_or in aio_ctx_prepare or atomic_add in aio_poll.
+     */
     smp_mb();
-    if (!ctx->dispatching) {
+    if (ctx->notify_me) {
         event_notifier_set(&ctx->notifier);
     }
 }
diff --git a/docs/aio_notify.promela b/docs/aio_notify.promela
index ad3f6f0..fccc7ee 100644
--- a/docs/aio_notify.promela
+++ b/docs/aio_notify.promela
@@ -1,5 +1,5 @@
 /*
- * This model describes the interaction between aio_set_dispatching()
+ * This model describes the interaction between ctx->notify_me
  * and aio_notify().
  *
  * Author: Paolo Bonzini <pbonzini@redhat.com>
@@ -14,57 +14,53 @@
  *     spin -a docs/aio_notify.promela
  *     gcc -O2 pan.c
  *     ./a.out -a
+ *
+ * To verify it (with a bug planted in the model):
+ *     spin -a -DBUG docs/aio_notify.promela
+ *     gcc -O2 pan.c
+ *     ./a.out -a
  */
 
 #define MAX   4
 #define LAST  (1 << (MAX - 1))
 #define FINAL ((LAST << 1) - 1)
 
-bool dispatching;
+bool notify_me;
 bool event;
 
-int req, done;
+int req;
+int done;
 
 active proctype waiter()
 {
-     int fetch, blocking;
+    int fetch;
 
-     do
-        :: done != FINAL -> {
-            // Computing "blocking" is separate from execution of the
-            // "bottom half"
-            blocking = (req == 0);
+    do
+        :: true -> {
+            notify_me++;
 
-            // This is our "bottom half"
+            if
+#ifndef BUG
+                :: (req > 0) -> skip;
+#endif
+                :: else ->
+                    // Wait for a nudge from the other side
+                    do
+                        :: event == 1 -> { event = 0; break; }
+                    od;
+            fi;
+
+            notify_me--;
+
             atomic { fetch = req; req = 0; }
             done = done | fetch;
-
-            // Wait for a nudge from the other side
-            do
-                :: event == 1 -> { event = 0; break; }
-                :: !blocking  -> break;
-            od;
-
-            dispatching = 1;
-
-            // If you are simulating this model, you may want to add
-            // something like this here:
-            //
-            //      int foo; foo++; foo++; foo++;
-            //
-            // This only wastes some time and makes it more likely
-            // that the notifier process hits the "fast path".
-
-            dispatching = 0;
         }
-        :: else -> break;
     od
 }
 
 active proctype notifier()
 {
     int next = 1;
-    int sets = 0;
 
     do
         :: next <= LAST -> {
@@ -74,8 +70,8 @@
 
             // aio_notify
             if
-                :: dispatching == 0 -> sets++; event = 1;
-                :: else             -> skip;
+                :: notify_me == 1 -> event = 1;
+                :: else           -> printf("Skipped event_notifier_set\n"); skip;
             fi;
 
             // Test both synchronous and asynchronous delivery
@@ -86,19 +82,12 @@
                 :: 1 -> skip;
             fi;
         }
-        :: else -> break;
     od;
-    printf("Skipped %d event_notifier_set\n", MAX - sets);
 }
 
-#define p (done == FINAL)
-
-never  {
-    do
-        :: 1                      // after an arbitrarily long prefix
-        :: p -> break             // p becomes true
-    od;
-    do
-        :: !p -> accept: break    // it then must remains true forever after
-    od
+never { /* [] done < FINAL */
+accept_init:
+        do
+        :: done < FINAL -> skip;
+        od;
 }
diff --git a/include/block/aio.h b/include/block/aio.h
index b46103e..be91e3f 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -63,10 +63,30 @@
      */
     int walking_handlers;
 
-    /* Used to avoid unnecessary event_notifier_set calls in aio_notify.
-     * Writes protected by lock or BQL, reads are lockless.
+    /* Used to avoid unnecessary event_notifier_set calls in aio_notify;
+     * accessed with atomic primitives.  If this field is 0, everything
+     * (file descriptors, bottom halves, timers) will be re-evaluated
+     * before the next blocking poll(), thus the event_notifier_set call
+     * can be skipped.  If it is non-zero, you may need to wake up a
+     * concurrent aio_poll or the glib main event loop, making
+     * event_notifier_set necessary.
+     *
+     * Bit 0 is reserved for GSource usage of the AioContext, and is 1
+     * between a call to aio_ctx_check and the next call to aio_ctx_dispatch.
+     * Bits 1-31 simply count the number of active calls to aio_poll
+     * that are in the prepare or poll phase.
+     *
+     * The GSource and aio_poll must use a different mechanism because
+     * there is no certainty that a call to GSource's prepare callback
+     * (via g_main_context_prepare) is indeed followed by check and
+     * dispatch.  It's not clear whether this would be a bug, but let's
+     * play safe and allow it---it will just cause extra calls to
+     * event_notifier_set until the next call to dispatch.
+     *
+     * Instead, the aio_poll calls include both the prepare and the
+     * dispatch phase, hence a simple counter is enough for them.
      */
-    bool dispatching;
+    uint32_t notify_me;
 
     /* lock to protect between bh's adders and deleter */
     QemuMutex bh_lock;
@@ -89,9 +109,6 @@
     QEMUTimerListGroup tlg;
 };
 
-/* Used internally to synchronize aio_poll against qemu_bh_schedule.  */
-void aio_set_dispatching(AioContext *ctx, bool dispatching);
-
 /**
  * aio_context_new: Allocate a new AioContext.
  *
diff --git a/tests/test-aio.c b/tests/test-aio.c
index e7bbb83..217e337 100644
--- a/tests/test-aio.c
+++ b/tests/test-aio.c
@@ -97,14 +97,6 @@
 
 /* Tests using aio_*.  */
 
-static void test_notify(void)
-{
-    g_assert(!aio_poll(ctx, false));
-    aio_notify(ctx);
-    g_assert(!aio_poll(ctx, true));
-    g_assert(!aio_poll(ctx, false));
-}
-
 typedef struct {
     QemuMutex start_lock;
     bool thread_acquired;
@@ -494,14 +486,6 @@
  *   works well, and that's what I am using.
  */
 
-static void test_source_notify(void)
-{
-    while (g_main_context_iteration(NULL, false));
-    aio_notify(ctx);
-    g_assert(g_main_context_iteration(NULL, true));
-    g_assert(!g_main_context_iteration(NULL, false));
-}
-
 static void test_source_flush(void)
 {
     g_assert(!g_main_context_iteration(NULL, false));
@@ -830,7 +814,6 @@
     while (g_main_context_iteration(NULL, false));
 
     g_test_init(&argc, &argv, NULL);
-    g_test_add_func("/aio/notify",                  test_notify);
     g_test_add_func("/aio/acquire",                 test_acquire);
     g_test_add_func("/aio/bh/schedule",             test_bh_schedule);
     g_test_add_func("/aio/bh/schedule10",           test_bh_schedule10);
@@ -845,7 +828,6 @@
     g_test_add_func("/aio/event/flush",             test_flush_event_notifier);
     g_test_add_func("/aio/timer/schedule",          test_timer_schedule);
 
-    g_test_add_func("/aio-gsource/notify",                  test_source_notify);
     g_test_add_func("/aio-gsource/flush",                   test_source_flush);
     g_test_add_func("/aio-gsource/bh/schedule",             test_source_bh_schedule);
     g_test_add_func("/aio-gsource/bh/schedule10",           test_source_bh_schedule10);