block: clean up bdrv_drain_all() throttling comments

Since cc0681c45430a1f1a4c2d06e9499b7775afc9a18 ("block: Enable the new
throttling code in the block layer.") bdrv_drain_all() no longer spins.
The code used to look as follows:

  do {
      busy = qemu_aio_wait();

      /* FIXME: We do not have timer support here, so this is effectively
       * a busy wait.
       */
      QTAILQ_FOREACH(bs, &bdrv_states, list) {
          while (qemu_co_enter_next(&bs->throttled_reqs)) {
              busy = true;
          }
      }
  } while (busy);

Note that throttle requests are kicked but I/O throttling limits are
still in effect.  The loop spins until the vm_clock time allows the
request to make progress and complete.

The new throttling code introduced bdrv_start_throttled_reqs().  This
function not only kicks throttled requests but also temporarily disables
throttling so requests can run.

The outdated FIXME comment can be removed.  Also drop the busy = true
assignment since we overwrite it immediately afterwards.

Reviewed-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 1 insertions, 6 deletions

diff --git a/block.c b/block.c
index 1c83ef1fab..13f001ad69 100644
--- a/block.c
+++ b/block.c
@@ -1557,13 +1557,8 @@ void bdrv_drain_all(void)
     BlockDriverState *bs;
 
     while (busy) {
-        /* FIXME: We do not have timer support here, so this is effectively
-         * a busy wait.
-         */
         QTAILQ_FOREACH(bs, &bdrv_states, list) {
-            if (bdrv_start_throttled_reqs(bs)) {
-                busy = true;
-            }
+            bdrv_start_throttled_reqs(bs);
         }
 
         busy = bdrv_requests_pending_all();