Re: [PATCH v2 11/15] block: Call drain callbacks only once

[Kevin Wolf]

Am 25.11.2022 um 15:59 hat Vladimir Sementsov-Ogievskiy geschrieben:
> On 11/18/22 20:41, Kevin Wolf wrote:
> > We only need to call both the BlockDriver's callback and the parent
> > callbacks when going from undrained to drained or vice versa. A second
> > drain section doesn't make a difference for the driver or the parent,
> > they weren't supposed to send new requests before and after the second
> > drain.
> > 
> > One thing that gets in the way is the 'ignore_bds_parents' parameter in
> > bdrv_do_drained_begin_quiesce() and bdrv_do_drained_end(): It means that
> > bdrv_drain_all_begin() increases bs->quiesce_counter, but does not
> > quiesce the parent through BdrvChildClass callbacks. If an additional
> > drain section is started now, bs->quiesce_counter will be non-zero, but
> > we would still need to quiesce the parent through BdrvChildClass in
> > order to keep things consistent (and unquiesce it on the matching
> > bdrv_drained_end(), even though the counter would reach 0 yet as long as
> 
> would reach -> would NOT reach
> 
> if I understand correctly)

Yes, you're right. Thanks for catching it. Fixing this while applying
the series.

Kevin

> > the bdrv_drain_all() section is still active).
> > 
> > Instead of keeping track of this, let's just get rid of the parameter.
> > It was introduced in commit 6cd5c9d7b2d as an optimisation so that
> > during bdrv_drain_all(), we wouldn't recursively drain all parents up to
> > the root for each node, resulting in quadratic complexity. As it happens,
> > calling the callbacks only once solves the same problem, so as of this
> > patch, we'll still have O(n) complexity and ignore_bds_parents is not
> > needed any more.
> > 
> > This patch only ignores the 'ignore_bds_parents' parameter. It will be
> > removed in a separate patch.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > Reviewed-by: Hanna Reitz <hreitz@redhat.com>
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> 
> 
> 
> 
> -- 
> Best regards,
> Vladimir
>