Re: [Qemu-block] [PATCH 08/19] block: Remove bdrv_drain_recurse()

[Paolo Bonzini]

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 <address@hidden>
> ---
>  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++;
>