On 11/04/2018 18:39, Kevin Wolf wrote:
> For bdrv_drain(), recursively waiting for child node requests is
> pointless because we didn't quiesce their parents, so new requests could
> come in anyway. Letting the function work only on a single node makes it
> more consistent.
>
> For subtree drains and drain_all, we already have the recursion in
> bdrv_do_drained_begin(), so the extra recursion doesn't add anything
> either.
>
> Remove the useless code.
This solves one of my questions, but leaves me the doubt about quadratic
complexity.
Paolo
> Signed-off-by: Kevin Wolf <kw...@redhat.com>
> ---
> block/io.c | 36 +++---------------------------------
> 1 file changed, 3 insertions(+), 33 deletions(-)
>
> diff --git a/block/io.c b/block/io.c
> index 0a778eeff4..f24f39c278 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -211,38 +211,6 @@ bool bdrv_drain_poll(BlockDriverState *bs, bool
> top_level)
> return atomic_read(&bs->in_flight);
> }
>
> -static bool bdrv_drain_recurse(BlockDriverState *bs)
> -{
> - BdrvChild *child, *tmp;
> - bool waited;
> -
> - /* Wait for drained requests to finish */
> - waited = BDRV_POLL_WHILE(bs, bdrv_drain_poll(bs, true));
> -
> - QLIST_FOREACH_SAFE(child, &bs->children, next, tmp) {
> - BlockDriverState *bs = child->bs;
> - bool in_main_loop =
> - qemu_get_current_aio_context() == qemu_get_aio_context();
> - assert(bs->refcnt > 0);
> - if (in_main_loop) {
> - /* In case the recursive bdrv_drain_recurse processes a
> - * block_job_defer_to_main_loop BH and modifies the graph,
> - * let's hold a reference to bs until we are done.
> - *
> - * IOThread doesn't have such a BH, and it is not safe to call
> - * bdrv_unref without BQL, so skip doing it there.
> - */
> - bdrv_ref(bs);
> - }
> - waited |= bdrv_drain_recurse(bs);
> - if (in_main_loop) {
> - bdrv_unref(bs);
> - }
> - }
> -
> - return waited;
> -}
> -
> static void bdrv_do_drained_begin(BlockDriverState *bs, bool recursive,
> BdrvChild *parent);
> static void bdrv_do_drained_end(BlockDriverState *bs, bool recursive,
> @@ -310,7 +278,9 @@ void bdrv_do_drained_begin(BlockDriverState *bs, bool
> recursive,
>
> bdrv_parent_drained_begin(bs, parent);
> bdrv_drain_invoke(bs, true);
> - bdrv_drain_recurse(bs);
> +
> + /* Wait for drained requests to finish */
> + BDRV_POLL_WHILE(bs, bdrv_drain_poll(bs, true));
>
> if (recursive) {
> bs->recursive_quiesce_counter++;
>
