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[Qemu-devel] [PATCH] win32: fix main-loop busy loop on socket/fd event

From: Marc-André Lureau
Subject: [Qemu-devel] [PATCH] win32: fix main-loop busy loop on socket/fd event
Date: Wed, 4 Jan 2017 21:59:36 +0100

Commit 05e514b1d4d5bd4209e2c8bbc76ff05c85a235f3 introduced an AIO
context optimization to avoid calling event_notifier_test_and_clear() on
ctx->notifier. On Windows, the same notifier is being used to wakeup the
wait on socket events (see commit

The ctx->notifier event is added to the gpoll sources in
aio_set_event_notifier(), aio_ctx_check() should clear the event
regardless of ctx->notified, since Windows sets the event by itself,
bypassing the aio->notified. This fixes qemu not clearing the event
resulting in a busy loop.

Paolo suggested to me on irc to call event_notifier_test_and_clear()
after select() >0 from aio-win32.c's aio_prepare. Unfortunately, not all
fds associated with ctx->notifiers are in AIO fd handlers set.
(qemu_set_nonblock() in util/oslib-win32.c calls qemu_fd_register()).

Signed-off-by: Marc-André Lureau <address@hidden>
 include/block/aio.h | 2 ++
 async.c             | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/include/block/aio.h b/include/block/aio.h
index ca551e346f..a6da135bf3 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -100,6 +100,7 @@ struct AioContext {
     int walking_bh;
+#ifndef _WIN32
     /* Used by aio_notify.
      * "notified" is used to avoid expensive event_notifier_test_and_clear
@@ -113,6 +114,7 @@ struct AioContext {
      * in the docs/aio_notify_accept.promela formal model.
     bool notified;
     EventNotifier notifier;
     /* Thread pool for performing work and receiving completion callbacks */
diff --git a/async.c b/async.c
index b2de360c23..8c2a68b6cc 100644
--- a/async.c
+++ b/async.c
@@ -329,15 +329,21 @@ void aio_notify(AioContext *ctx)
     if (ctx->notify_me) {
+#ifndef _WIN32
         atomic_mb_set(&ctx->notified, true);
 void aio_notify_accept(AioContext *ctx)
+#ifndef _WIN32
     if (atomic_xchg(&ctx->notified, false)) {
+#ifndef _WIN32
 static void aio_timerlist_notify(void *opaque)

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