Re: [Qemu-devel] [PATCH v2 05/12] block: Reduce (un)drains when replacing a child

[Kevin Wolf <kwolf@redhat.com>]

Am 03.07.2019 um 19:28 hat Max Reitz geschrieben:
> Currently, bdrv_replace_child_noperm() undrains the parent until it is
> completely undrained, then re-drains it after attaching the new child
> node.
> 
> This is a problem with bdrv_drop_intermediate(): We want to keep the
> whole subtree drained, including parents, while the operation is
> under way.  bdrv_replace_child_noperm() breaks this by allowing every
> parent to become unquiesced briefly, and then redraining it.
> 
> In fact, there is no reason why the parent should become unquiesced and
> be allowed to submit requests to the new child node if that new node is
> supposed to be kept drained.  So if anything, we have to drain the
> parent before detaching the old child node.  Conversely, we have to
> undrain it only after attaching the new child node.
> 
> Thus, change the whole drain algorithm here: Calculate the number of
> times we have to drain/undrain the parent before replacing the child
> node then drain it (if necessary), replace the child node, and then
> undrain it.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>

I think this looks good (and fixes a bug that Denis Plotnikov reported
before while trying to fix IDE submitting requests in a drained
section), but I would be even more confident if we could add tests for
the various cases to tests/test-bdrv-drain.c.

Kevin