[Qemu-devel] [PATCH 3/8] aio: Fix return value of aio_poll()

[Stefan Hajnoczi <stefanha@redhat.com>]

From: Kevin Wolf <kwolf@redhat.com>

aio_poll() must return true if any work is still pending, even if it
didn't make progress, so that bdrv_drain_all() doesn't stop waiting too
early. The possibility of stopping early occasionally lead to a failed
assertion in bdrv_drain_all(), when some in-flight request was missed
and the function didn't really drain all requests.

In order to make that change, the return value as specified in the
function comment must change for blocking = false; fortunately, the
return value of blocking = false callers is only used in test cases, so
this change shouldn't cause any trouble.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 aio-posix.c         | 3 ++-
 aio-win32.c         | 3 ++-
 include/block/aio.h | 6 ++----
 tests/test-aio.c    | 4 ++--
 4 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/aio-posix.c b/aio-posix.c
index 88d09e1..fe4dbb4 100644
--- a/aio-posix.c
+++ b/aio-posix.c
@@ -264,5 +264,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
         }
     }
 
-    return progress;
+    assert(progress || busy);
+    return true;
 }
diff --git a/aio-win32.c b/aio-win32.c
index f5ea027..38723bf 100644
--- a/aio-win32.c
+++ b/aio-win32.c
@@ -214,5 +214,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
         events[ret - WAIT_OBJECT_0] = events[--count];
     }
 
-    return progress;
+    assert(progress || busy);
+    return true;
 }
diff --git a/include/block/aio.h b/include/block/aio.h
index 0933f05..8eda924 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -173,16 +173,14 @@ bool aio_pending(AioContext *ctx);
  * aio as a result of executing I/O completion or bh callbacks.
  *
  * If there is no pending AIO operation or completion (bottom half),
- * return false.  If there are pending bottom halves, return true.
+ * return false.  If there are pending AIO operations of bottom halves,
+ * return true.
  *
  * If there are no pending bottom halves, but there are pending AIO
  * operations, it may not be possible to make any progress without
  * blocking.  If @blocking is true, this function will wait until one
  * or more AIO events have completed, to ensure something has moved
  * before returning.
- *
- * If @blocking is false, this function will also return false if the
- * function cannot make any progress without blocking.
  */
 bool aio_poll(AioContext *ctx, bool blocking);
 
diff --git a/tests/test-aio.c b/tests/test-aio.c
index e4ebef7..c173870 100644
--- a/tests/test-aio.c
+++ b/tests/test-aio.c
@@ -315,13 +315,13 @@ static void test_wait_event_notifier_noflush(void)
     event_notifier_set(&data.e);
     g_assert(aio_poll(ctx, false));
     g_assert_cmpint(data.n, ==, 1);
-    g_assert(!aio_poll(ctx, false));
+    g_assert(aio_poll(ctx, false));
     g_assert_cmpint(data.n, ==, 1);
 
     event_notifier_set(&data.e);
     g_assert(aio_poll(ctx, false));
     g_assert_cmpint(data.n, ==, 2);
-    g_assert(!aio_poll(ctx, false));
+    g_assert(aio_poll(ctx, false));
     g_assert_cmpint(data.n, ==, 2);
 
     event_notifier_set(&dummy.e);
-- 
1.8.0.2