[Qemu-devel] [RFC 01/13] block: stop relying on io_flush() in bdrv_drain_all()

[Stefan Hajnoczi <stefanha@redhat.com>]

If a block driver has no file descriptors to monitor but there are still
active requests, it can return 1 from .io_flush().  This is used to spin
during synchronous I/O.

Stop relying on .io_flush() and instead check
QLIST_EMPTY(&bs->tracked_requests) to decide whether there are active
requests.

This is the first step in removing .io_flush() so that event loops no
longer need to have the concept of synchronous I/O.  Eventually we may
be able to kill synchronous I/O completely by running everything in a
coroutine, but that is future work.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/block.c b/block.c
index 602d8a4..93d676b 100644
--- a/block.c
+++ b/block.c
@@ -1360,6 +1360,22 @@ void bdrv_close_all(void)
     }
 }
 
+/* Check if any requests are in-flight (including throttled requests) */
+static bool bdrv_requests_pending(void)
+{
+    BlockDriverState *bs;
+
+    QTAILQ_FOREACH(bs, &bdrv_states, list) {
+        if (!QLIST_EMPTY(&bs->tracked_requests)) {
+            return true;
+        }
+        if (!qemu_co_queue_empty(&bs->throttled_reqs)) {
+            return true;
+        }
+    }
+    return false;
+}
+
 /*
  * Wait for pending requests to complete across all BlockDriverStates
  *
@@ -1375,26 +1391,18 @@ void bdrv_close_all(void)
 void bdrv_drain_all(void)
 {
     BlockDriverState *bs;
-    bool busy;
-
-    do {
-        busy = qemu_aio_wait();
 
+    while (bdrv_requests_pending()) {
         /* FIXME: We do not have timer support here, so this is effectively
          * a busy wait.
          */
         QTAILQ_FOREACH(bs, &bdrv_states, list) {
             if (!qemu_co_queue_empty(&bs->throttled_reqs)) {
                 qemu_co_queue_restart_all(&bs->throttled_reqs);
-                busy = true;
             }
         }
-    } while (busy);
 
-    /* If requests are still pending there is a bug somewhere */
-    QTAILQ_FOREACH(bs, &bdrv_states, list) {
-        assert(QLIST_EMPTY(&bs->tracked_requests));
-        assert(qemu_co_queue_empty(&bs->throttled_reqs));
+        qemu_aio_wait();
     }
 }
 
-- 
1.8.1.4