block: Reduce (un)drains when replacing a child
commitdebc29276714756f278d98ae1929d7813f04030c
authorMax Reitz <mreitz@redhat.com>
Mon, 22 Jul 2019 13:33:44 +0000 (22 15:33 +0200)
committerKevin Wolf <kwolf@redhat.com>
Fri, 16 Aug 2019 08:25:16 +0000 (16 10:25 +0200)
tree0d67f0d2ccfa2946e4a868b7674a0b8c4a6f0e67
parent637d54a5f3a1965fa6aea9e41c4f84abf7a36de9
block: Reduce (un)drains when replacing a child

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>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
block.c