Re: [Qemu-devel] [PATCH V3 1/4] block: drop bs_snapshots global variable

[Wenchao Xia]

于 2013-5-27 23:25, Kevin Wolf 写道:
> Am 25.05.2013 um 05:09 hat Wenchao Xia geschrieben:
> > From: Stefan Hajnoczi <address@hidden>
> >
> > The bs_snapshots global variable points to the BlockDriverState which
> > will be used to save vmstate.  This is really a savevm.c concept but was
> > moved into block.c:bdrv_snapshots() when it became clear that hotplug
> > could result in a dangling pointer.
> >
> > While auditing the block layer's global state I came upon bs_snapshots
> > and realized that a variable is not necessary here.  Simply find the
> > first BlockDriverState capable of internal snapshots each time this is
> > needed.
> >
> > The behavior of bdrv_snapshots() is preserved across hotplug because new
> > drives are always appended to the bdrv_states list.  This means that
> > calling the new find_vmstate_bs() function is idempotent - it returns
> > the same BlockDriverState unless it was hot-unplugged.
> >
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
> > Reviewed-by: Eric Blake <address@hidden>
> > Reviewed-by: Wenchao Xia <address@hidden>
> > Signed-off-by: Wenchao Xia <address@hidden>
> > ---
> >   block.c               |   28 ----------------------------
> >   include/block/block.h |    1 -
> >   savevm.c              |   19 +++++++++++++++----
> >   3 files changed, 15 insertions(+), 33 deletions(-)
> >
> > diff --git a/block.c b/block.c
> > index 3f87489..478a3b2 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -99,9 +99,6 @@ static QTAILQ_HEAD(, BlockDriverState) bdrv_states =
> >   static QLIST_HEAD(, BlockDriver) bdrv_drivers =
> >       QLIST_HEAD_INITIALIZER(bdrv_drivers);
> >
> > -/* The device to use for VM snapshots */
> > -static BlockDriverState *bs_snapshots;
> > -
> >   /* If non-zero, use only whitelisted block drivers */
> >   static int use_bdrv_whitelist;
> >
> > @@ -1357,9 +1354,6 @@ void bdrv_close(BlockDriverState *bs)
> >       notifier_list_notify(&bs->close_notifiers, bs);
> >
> >       if (bs->drv) {
> > -        if (bs == bs_snapshots) {
> > -            bs_snapshots = NULL;
> > -        }
> >           if (bs->backing_hd) {
> >               bdrv_delete(bs->backing_hd);
> >               bs->backing_hd = NULL;
> > @@ -1591,7 +1585,6 @@ void bdrv_delete(BlockDriverState *bs)
> >
> >       bdrv_close(bs);
> >
> > -    assert(bs != bs_snapshots);
> >       g_free(bs);
> >   }
> >
> > @@ -1635,9 +1628,6 @@ void bdrv_set_dev_ops(BlockDriverState *bs, const 
> > BlockDevOps *ops,
> >   {
> >       bs->dev_ops = ops;
> >       bs->dev_opaque = opaque;
> > -    if (bdrv_dev_has_removable_media(bs) && bs == bs_snapshots) {
> > -        bs_snapshots = NULL;
> > -    }
>
> This hunk isn't replaced by any other code. If I understand correctly
> what it's doing, it prevented you from saving the VM state to a
> removable device, which would be allowed after this patch.
>
> Is this a change we want to make? Why isn't it described in the commit
> message?
  How about adding it back in find_vmstate_bs()?

> Kevin


--
Best Regards

Wenchao Xia