Re: [Qemu-devel] [PATCH untested] mirror: start drained section earlier

[Fam Zheng <famz@redhat.com>]

On Wed, 12/16 19:33, Paolo Bonzini wrote:
> Ensure that the guest does not write anything to disk after cnt is
> read for the final time.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>         Untested.
> 
>  block/mirror.c | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 0e8f556..e57c246 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -562,8 +562,18 @@ static void coroutine_fn mirror_run(void *opaque)
>               * mirror_populate runs.
>               */
>              trace_mirror_before_drain(s, cnt);
> -            bdrv_drain(bs);
> +            bdrv_drained_begin(s->common.bs);
>              cnt = bdrv_get_dirty_count(s->dirty_bitmap);
> +
> +            if (cnt == 0) {
> +                /* The two disks are in sync.  Exit and report successful
> +                 * completion.
> +                 */
> +                assert(QLIST_EMPTY(&bs->tracked_requests));
> +                s->common.cancelled = false;
> +                break;
> +            }
> +            bdrv_drained_end(s->common.bs);
>          }
>  
>          ret = 0;
> @@ -576,13 +586,6 @@ static void coroutine_fn mirror_run(void *opaque)
>          } else if (!should_complete) {
>              delay_ns = (s->in_flight == 0 && cnt == 0 ? SLICE_TIME : 0);
>              block_job_sleep_ns(&s->common, QEMU_CLOCK_REALTIME, delay_ns);
> -        } else if (cnt == 0) {
> -            /* The two disks are in sync.  Exit and report successful
> -             * completion.
> -             */
> -            assert(QLIST_EMPTY(&bs->tracked_requests));
> -            s->common.cancelled = false;
> -            break;
>          }
>          last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>      }
> @@ -608,9 +611,6 @@ immediate_exit:
>  
>      data = g_malloc(sizeof(*data));
>      data->ret = ret;
> -    /* Before we switch to target in mirror_exit, make sure data doesn't
> -     * change. */
> -    bdrv_drained_begin(s->common.bs);
>      block_job_defer_to_main_loop(&s->common, mirror_exit, data);
>  }

bdrv_drained_begin was unconditional to balance with the one in mirror_exit,
but now it is only called if cnt == 0. Squashing this in fixes it:

diff --git a/block/mirror.c b/block/mirror.c
index e57c246..a814929 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -62,6 +62,7 @@ typedef struct MirrorBlockJob {
     int ret;
     bool unmap;
     bool waiting_for_io;
+    bool drained_begin;
 } MirrorBlockJob;
 
 typedef struct MirrorOp {
@@ -355,6 +356,7 @@ static void mirror_exit(BlockJob *job, void *opaque)
     MirrorExitData *data = opaque;
     AioContext *replace_aio_context = NULL;
     BlockDriverState *src = s->common.bs;
+    bool drained_begin = s->drained_begin;
 
     /* Make sure that the source BDS doesn't go away before we called
      * block_job_completed(). */
@@ -388,7 +390,9 @@ static void mirror_exit(BlockJob *job, void *opaque)
     bdrv_unref(s->target);
     block_job_completed(&s->common, data->ret);
     g_free(data);
-    bdrv_drained_end(src);
+    if (drained_begin) {
+        bdrv_drained_end(src);
+    }
     bdrv_unref(src);
 }
 
@@ -571,6 +575,7 @@ static void coroutine_fn mirror_run(void *opaque)
                  */
                 assert(QLIST_EMPTY(&bs->tracked_requests));
                 s->common.cancelled = false;
+                s->drained_begin = true;
                 break;
             }
             bdrv_drained_end(s->common.bs);