On Wed, 09/13 20:18, Max Reitz wrote:
> Drainined a BDS child may lead to both the original BDS and/or its other
> children being deleted (e.g. if the original BDS represents a block
> job). We should prepare for this in both bdrv_drain_recurse() and
> bdrv_drained_begin() by monitoring whether the BDS we are about to drain
> still exists at all.
Can the deletion happen when IOThread calls
bdrv_drain_recurse/bdrv_drained_begin? If not, is it enough to do
...
if (in_main_loop) {
bdrv_ref(bs);
}
...
if (in_main_loop) {
bdrv_unref(bs);
}
to protect the main loop case? So the BdrvDeletedStatus state is not needed.
Fam
>
> Signed-off-by: Max Reitz <mre...@redhat.com>
> ---
> block/io.c | 72
> +++++++++++++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 52 insertions(+), 20 deletions(-)
>
> diff --git a/block/io.c b/block/io.c
> index 4378ae4c7d..8ec1a564ad 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -182,33 +182,57 @@ static void bdrv_drain_invoke(BlockDriverState *bs)
>
> static bool bdrv_drain_recurse(BlockDriverState *bs)
> {
> - BdrvChild *child, *tmp;
> + BdrvChild *child;
> bool waited;
> + struct BDSToDrain {
> + BlockDriverState *bs;
> + BdrvDeletedStatus del_stat;
> + QLIST_ENTRY(BDSToDrain) next;
> + };
> + QLIST_HEAD(, BDSToDrain) bs_list = QLIST_HEAD_INITIALIZER(bs_list);
> + bool in_main_loop =
> + qemu_get_current_aio_context() == qemu_get_aio_context();
>
> waited = BDRV_POLL_WHILE(bs, atomic_read(&bs->in_flight) > 0);
>
> /* Ensure any pending metadata writes are submitted to bs->file. */
> bdrv_drain_invoke(bs);
>
> - 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);
> + /* Draining children may result in other children being removed and maybe
> + * even deleted, so copy the children list first */
> + QLIST_FOREACH(child, &bs->children, next) {
> + struct BDSToDrain *bs2d = g_new0(struct BDSToDrain, 1);
> +
> + bs2d->bs = child->bs;
> + QLIST_INSERT_HEAD(&bs->deleted_status, &bs2d->del_stat, next);
> +
> + QLIST_INSERT_HEAD(&bs_list, bs2d, next);
> + }
> +
> + while (!QLIST_EMPTY(&bs_list)) {
> + struct BDSToDrain *bs2d = QLIST_FIRST(&bs_list);
> + QLIST_REMOVE(bs2d, next);
> +
> + if (!bs2d->del_stat.deleted) {
> + QLIST_REMOVE(&bs2d->del_stat, next);
> +
> + 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 the BDS 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(bs2d->bs);
> + }
> + waited |= bdrv_drain_recurse(bs2d->bs);
> + if (in_main_loop) {
> + bdrv_unref(bs2d->bs);
> + }
> }
> +
> + g_free(bs2d);
> }
>
> return waited;
> @@ -252,17 +276,25 @@ static void coroutine_fn
> bdrv_co_yield_to_drain(BlockDriverState *bs)
>
> void bdrv_drained_begin(BlockDriverState *bs)
> {
> + BdrvDeletedStatus del_stat = { .deleted = false };
> +
> if (qemu_in_coroutine()) {
> bdrv_co_yield_to_drain(bs);
> return;
> }
>
> + QLIST_INSERT_HEAD(&bs->deleted_status, &del_stat, next);
> +
> if (atomic_fetch_inc(&bs->quiesce_counter) == 0) {
> aio_disable_external(bdrv_get_aio_context(bs));
> bdrv_parent_drained_begin(bs);
> }
>
> - bdrv_drain_recurse(bs);
> + if (!del_stat.deleted) {
> + QLIST_REMOVE(&del_stat, next);
> +
> + bdrv_drain_recurse(bs);
> + }
> }
>
> void bdrv_drained_end(BlockDriverState *bs)
> --
> 2.13.5
>
