[Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close()

[Max Reitz <mreitz@redhat.com>]

When nbd_close() is called from a coroutine, the connection_co never
gets to run, and thus nbd_teardown_connection() hangs.

This is because aio_co_enter() only puts the connection_co into the main
coroutine's wake-up queue, so this main coroutine needs to yield and
reschedule itself to let the connection_co run.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/nbd.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/block/nbd.c b/block/nbd.c
index 81edabbf35..b83b6cd43e 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -135,7 +135,17 @@ static void nbd_teardown_connection(BlockDriverState *bs)
     qio_channel_shutdown(s->ioc,
                          QIO_CHANNEL_SHUTDOWN_BOTH,
                          NULL);
-    BDRV_POLL_WHILE(bs, s->connection_co);
+
+    if (qemu_in_coroutine()) {
+        /* Let our caller poll and just yield until connection_co is done */
+        while (s->connection_co) {
+            aio_co_schedule(qemu_get_current_aio_context(),
+                            qemu_coroutine_self());
+            qemu_coroutine_yield();
+        }
+    } else {
+        BDRV_POLL_WHILE(bs, s->connection_co);
+    }
 
     nbd_client_detach_aio_context(bs);
     object_unref(OBJECT(s->sioc));
-- 
2.21.0