block: stop relying on io_flush() in bdrv_drain_all()
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.
Note this patch moves bs->throttled_reqs initialization to bdrv_new() so
that bdrv_requests_pending(bs) can safely access it. In practice bs is
g_malloc0() so the memory is already zeroed but it's safer to initialize
the queue properly.
We also need to fix up block/stream.c:close_unused_images() to prevent
traversing a dangling pointer while it rearranges the backing file
chain. This is necessary since the new bdrv_drain_all() traverses the
backing file chain.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/block/stream.c b/block/stream.c
index 7fe9e48..db49b4d 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -57,6 +57,11 @@
BlockDriverState *intermediate;
intermediate = top->backing_hd;
+ /* Must assign before bdrv_delete() to prevent traversing dangling pointer
+ * while we delete backing image instances.
+ */
+ top->backing_hd = base;
+
while (intermediate) {
BlockDriverState *unused;
@@ -70,7 +75,6 @@
unused->backing_hd = NULL;
bdrv_delete(unused);
}
- top->backing_hd = base;
}
static void coroutine_fn stream_run(void *opaque)
