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

[Vladimir Sementsov-Ogievskiy]

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)

> 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