fix qemu_aio_flush

qemu_aio_wait by invoking the bh or one of the aio completion
callbacks, could end up submitting new pending aio, breaking the
invariant that qemu_aio_flush returns only when no pending aio is
outstanding (possibly a problem for migration as such).

(cherry picked from commit 986c28d655dc9196c9c426c667c1764bd3d6d5bd)

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Kevin Wolf <kwolf@redhat.com>

2 files changed, 10 insertions, 5 deletions

diff --git a/aio.c b/aio.c
index 11fbb6c0c5..dc9b85d167 100644
--- a/aio.c
+++ b/aio.c
@@ -103,11 +103,15 @@ void qemu_aio_flush(void)
     do {
         ret = 0;
 
+	/*
+	 * If there are pending emulated aio start them now so flush
+	 * will be able to return 1.
+	 */
+        qemu_aio_wait();
+
         LIST_FOREACH(node, &aio_handlers, node) {
             ret |= node->io_flush(node->opaque);
         }
-
-        qemu_aio_wait();
     } while (ret > 0);
 }
 
diff --git a/qemu-aio.h b/qemu-aio.h
index 79678293ef..f262344af3 100644
--- a/qemu-aio.h
+++ b/qemu-aio.h
@@ -24,9 +24,10 @@ typedef int (AioFlushHandler)(void *opaque);
  * outstanding AIO operations have been completed or cancelled. */
 void qemu_aio_flush(void);
 
-/* Wait for a single AIO completion to occur.  This function will until a
- * single AIO opeartion has completed.  It is intended to be used as a looping
- * primative when simulating synchronous IO based on asynchronous IO. */
+/* Wait for a single AIO completion to occur.  This function will wait
+ * until a single AIO event has completed and it will ensure something
+ * has moved before returning. This can issue new pending aio as
+ * result of executing I/O completion or bh callbacks. */
 void qemu_aio_wait(void);
 
 /* Register a file descriptor and associated callbacks.  Behaves very similarly