Re: [Qemu-devel] [PATCH 05/10] snapshot: create bdrv_all_find_snapshot helper

[Stefan Hajnoczi]

On Mon, Nov 16, 2015 at 06:24:36PM +0300, Denis V. Lunev wrote:
> +int bdrv_all_find_snapshot(const char *name, bool skip_read_only,
> +                           BlockDriverState **first_bad_bs)
> +{
> +    QEMUSnapshotInfo sn;
> +    int err = 0;
> +    BlockDriverState *bs = NULL;
> +
> +    while (err == 0 && (bs = bdrv_next(bs))) {
> +        AioContext *ctx = bdrv_get_aio_context(bs);
> +
> +        if (skip_read_only &&
> +                (!bdrv_is_inserted(bs) || bdrv_is_read_only(bs))) {

These must be called with AioContext acquired.

> +            continue;
> +        }
> +
> +        aio_context_acquire(ctx);
> +        if (bdrv_can_snapshot(bs)) {

The !bdrv_is_inserted(bs) || bdrv_is_read_only(bs) checks above are
redundant since bdrv_can_snapshot() checks too:

  int bdrv_can_snapshot(BlockDriverState *bs)
  {
      BlockDriver *drv = bs->drv;
      if (!drv || !bdrv_is_inserted(bs) || bdrv_is_read_only(bs)) {
          return 0;
      }

It also means that the "skip_read_only" name is inaccurate.  Read-only
drives are always skipped, regardless of skip_read_only's value.

The skip_read_only argument can be dropped and the earlier
!bdrv_is_inserted(bs) || bdrv_is_read_only(bs) check can be dropped too.

> @@ -2168,21 +2157,7 @@ void hmp_info_snapshots(Monitor *mon, const QDict 
> *qdict)
>      available_snapshots = g_new0(int, nb_sns);
>      total = 0;
>      for (i = 0; i < nb_sns; i++) {
> -        sn = &sn_tab[i];
> -        available = 1;
> -        bs1 = NULL;
> -
> -        while ((bs1 = bdrv_next(bs1))) {
> -            if (bdrv_can_snapshot(bs1) && bs1 != bs) {
> -                ret = bdrv_snapshot_find(bs1, sn_info, sn->id_str);
> -                if (ret < 0) {
> -                    available = 0;
> -                    break;
> -                }
> -            }
> -        }
> -
> -        if (available) {
> +        if (bdrv_all_find_snapshot(sn_tab[i].id_str, false, &bs1) == 0) {

bdrv_all_find_snapshot() doesn't do the bs1 != bs exclusion so the new
code behaves differently from the old code.  That seems like a bug.

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