block: Dec. drained_end_counter before bdrv_wakeup

Decrementing drained_end_counter after bdrv_dec_in_flight() (which in
turn invokes bdrv_wakeup() and thus aio_wait_kick()) is not very clever.
We should decrement it beforehand, so that any waiting aio_poll() that
is woken by bdrv_dec_in_flight() sees the decremented
drained_end_counter.

Because the time window between decrementing drained_end_counter and
aio_wait_kick() is very small, I cannot supply a reliable regression
test.  However, running e.g. the /bdrv-drain/blockjob/iothread/drain_all
test in test-bdrv-drain has a small chance of hanging without this
patch (about 1/200 or so; it gets to nearly 100 % if you add e.g. an
fputc(' ', stderr); after the bdrv_dec_in_flight()).

Fixes: e037c09
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190722133054.21781-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/io.c

@@ -217,13 +217,12 @@ static void coroutine_fn bdrv_drain_invoke_entry(void *opaque)
         bs->drv->bdrv_co_drain_end(bs);
     }
 
-    /* Set data->done before reading bs->wakeup.  */
+    /* Set data->done and decrement drained_end_counter before bdrv_wakeup() */
     atomic_mb_set(&data->done, true);
-    bdrv_dec_in_flight(bs);
-
     if (!data->begin) {
         atomic_dec(data->drained_end_counter);
     }
+    bdrv_dec_in_flight(bs);
 
     g_free(data);
 }