Re: [Qemu-devel] [PATCH 13/16] block: Implement bdrv_append() without bdrv_swap()

[Kevin Wolf <kwolf@redhat.com>]

Am 18.09.2015 um 13:45 hat Alberto Garcia geschrieben:
> On Thu 17 Sep 2015 03:48:17 PM CEST, Kevin Wolf wrote:
> 
> > +static void swap_feature_fields(BlockDriverState *bs_top,
> > +                                BlockDriverState *bs_new)
> > +{
> > +    BlockDriverState tmp;
> > +
> > +    bdrv_move_feature_fields(&tmp, bs_top);
> > +    bdrv_move_feature_fields(bs_top, bs_new);
> > +    bdrv_move_feature_fields(bs_new, &tmp);
> > +
> > +    assert(!bs_new->io_limits_enabled);
> > +    if (bs_top->io_limits_enabled) {
> > +        bdrv_io_limits_enable(bs_new, throttle_group_get_name(bs_top));
> > +        bdrv_io_limits_disable(bs_top);
> > +    }
> > +}
> 
> I would use 'if (bs_top->throttle_state != NULL)' instead. That pointer
> is set if and only if the BDS has I/O limits set.
> 
> bs->io_limits_enabled can be set to false in order to bypass the limits
> temporarily without removing the BDS's throttling settings (e.g in
> bdrv_read_unthrottled()).
> 
> I actually think that we can get rid of io_limits_enabled in several
> places (if not completely), but I will take care of that in a separate
> patch.
> 
> The only scenario that could cause problems is if bs->throttle_state is
> set but bs->io_limits_enabled is false when swap_feature_fields() is
> called, but I don't think that's possible in this case.

So something like this? (Specifically, is the assertion right?)

    assert(!bs_new->throttle_state);
    if (bs_top->throttle_state) {
        assert(bs_top->io_limits_enabled);
        bdrv_io_limits_enable(bs_new, throttle_group_get_name(bs_top));
        bdrv_io_limits_disable(bs_top);
    }

If the assertion doesn't hold true, I guess we would need to call
throttle_group_register() directly for the cases where io_limits_enabled
wasn't true.

Kevin