1 files changed, 23 insertions, 6 deletions
diff --git a/include/block/aio.h b/include/block/aio.h
index b46103ece7..be91e3f701 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -63,10 +63,30 @@ struct AioContext {
      */
     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 @@ struct AioContext {
     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.
  *