I'm aware that exactly nobody has been looking forward to a series with this title, but it has to be. The way drain works means that we need to poll in bdrv_replace_child_noperm() and that makes things rather messy with Emanuele's multiqueue work because you must not poll while you hold the graph lock.
The other reason why it has to be is that drain is way too complex and there are too many different cases. Some simplification like this will hopefully make it considerably more maintainable. The diffstat probably tells something, too. There are roughly speaking three parts in this series: 1. Make BlockDriver.bdrv_drained_begin/end() non-coroutine_fn again, which allows us to not poll on bdrv_drained_end() any more. 2. Remove subtree drains. They are a considerable complication in the whole drain machinery (in particular, they require polling in the BdrvChildClass.attach/detach() callbacks that are called during bdrv_replace_child_noperm()) and none of their users actually has a good reason to use them. 3. Finally get rid of polling in bdrv_replace_child_noperm() by requiring that the child is already drained by the caller and calling callbacks only once and not again for every nested drain section. If necessary, a prefix of this series can be merged that covers only the first or the first two parts and it would still make sense. Kevin Wolf (13): qed: Don't yield in bdrv_qed_co_drain_begin() test-bdrv-drain: Don't yield in .bdrv_co_drained_begin/end() block: Revert .bdrv_drained_begin/end to non-coroutine_fn block: Remove drained_end_counter block: Inline bdrv_drain_invoke() block: Drain invidual nodes during reopen block: Don't use subtree drains in bdrv_drop_intermediate() stream: Replace subtree drain with a single node drain block: Remove subtree drains block: Call drain callbacks only once block: Remove ignore_bds_parents parameter from drain functions block: Don't poll in bdrv_replace_child_noperm() block: Remove poll parameter from bdrv_parent_drained_begin_single() include/block/block-global-state.h | 3 + include/block/block-io.h | 52 +--- include/block/block_int-common.h | 17 +- include/block/block_int-io.h | 12 - block.c | 132 ++++++----- block/block-backend.c | 4 +- block/io.c | 281 ++++------------------ block/qed.c | 24 +- block/replication.c | 6 - block/stream.c | 20 +- block/throttle.c | 6 +- blockdev.c | 13 - blockjob.c | 2 +- tests/unit/test-bdrv-drain.c | 369 +++++++---------------------- 14 files changed, 270 insertions(+), 671 deletions(-) -- 2.38.1
