Re: [Qemu-devel] [PATCH for-3.1 1/2] block: Don't inactivate children before parents

[Kevin Wolf <kwolf@redhat.com>]

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Am 26.11.2018 um 13:05 hat Max Reitz geschrieben:
> On 26.11.18 12:28, Kevin Wolf wrote:
> > bdrv_child_cb_inactivate() asserts that parents are already inactive
> > when children get inactivated. This precondition is necessary because
> > parents could still issue requests in their inactivation code.
> > 
> > When block nodes are created individually with -blockdev, all of them
> > are monitor owned and will be returned by bdrv_next() in an undefined
> > order (in practice, in the order of their creation, which is usually
> > children before parents), which obviously fails the assertion.
> > 
> > This patch fixes the ordering by skipping nodes with still active
> > parents in bdrv_inactivate_recurse() because we know that they will be
> > covered by recursion when the last active parent becomes inactive.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >  block.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> > 
> > diff --git a/block.c b/block.c
> > index 5ba3435f8f..0569275e31 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -4612,6 +4612,22 @@ void bdrv_invalidate_cache_all(Error **errp)
> >      }
> >  }
> >  
> > +static bool bdrv_has_active_bds_parent(BlockDriverState *bs)
> > +{
> > +    BdrvChild *parent;
> > +
> > +    QLIST_FOREACH(parent, &bs->parents, next_parent) {
> > +        if (parent->role->parent_is_bds) {
> > +            BlockDriverState *parent_bs = parent->opaque;
> > +            if (!(parent_bs->open_flags & BDRV_O_INACTIVE)) {
> > +                return true;
> > +            }
> > +        }
> > +    }
> 
> Now I see why you say this might make sense as a permission.

You do? To be honest, now that I found this solution, I don't think a
permission makes much sense any more, because you would have the same
loop, and you would only be checking a different flag in the end.

> > +
> > +    return false;
> > +}
> > +
> >  static int bdrv_inactivate_recurse(BlockDriverState *bs,
> >                                     bool setting_flag)
> >  {
> > @@ -4622,6 +4638,12 @@ static int bdrv_inactivate_recurse(BlockDriverState *bs,
> >          return -ENOMEDIUM;
> >      }
> >  
> > +    /* Make sure that we don't inactivate a child before its parent.
> > +     * It will be covered by recursion from the yet active parent. */
> > +    if (bdrv_has_active_bds_parent(bs)) {
> > +        return 0;
> > +    }
> > +
> 
> Hm.  Wouldn't it make more sense to always return early when there are
> any BDS parents?  Because if there are any BDS parents and none of them
> are active (anymore), then this child will have been inactivated
> already, and we can save ourselves the trouble of going through the rest
> of the function again.

I don't quite follow... If we always return early no matter whether
there is an active parent, who will have inactivated the child?

After trying to write up why you're wrong, I think there are two cases
and both of us have a point:

1. bdrv_next() enumerates the child node first and then the last BDS
   parent. This is what this patch fixes.

   It will inactivate the child exactly once, at the time that the last
   parent has become inactive (and recursively calls this function for
   each of its children). If you remove that one inactivation, too,
   children won't be inactivated at all.

2. bdrv_next() enumerates the last BDS parent first and then the child.
   This is unlikely and might even be impossible today, but once we
   expose bdrv_reopen() in QMP and let the user reconfigure the edges,
   it probably becomes possible.

   In this case, even after my patch we inactivate drivers twice. Maybe
   we should just return early if BDRV_O_INACTIVE is already set. What
   makes me kind of unsure is that we already test for this flag, but
   only for part of the function.

   Any ideas how to test this? Can we create such a situation today?

> Do drivers support multiple calls to .bdrv_in_activate() at all?

They were probably not designed for that... Not sure if there was ever a
commit where we used to call them multiple times without failing the
assertion first.

Kevin

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