Re: [Qemu-devel] [Qemu-block] [PATCH v3 01/16] block: BDS deletion during bdrv_drain_recurse

[Jeff Cody <jcody@redhat.com>]

On Wed, Feb 28, 2018 at 07:04:52PM +0100, Max Reitz wrote:
> Draining a BDS child may lead to other children of the same parent being
> detached and/or deleted.  We should prepare for the former case (by
> copying the children list before iterating through it) and prevent the
> latter (by bdrv_ref()'ing all nodes if we are in the main loop).
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/io.c | 40 ++++++++++++++++++++++++++++++----------
>  1 file changed, 30 insertions(+), 10 deletions(-)
> 
> diff --git a/block/io.c b/block/io.c
> index 4d3d1f640a..ead12c4136 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -189,31 +189,51 @@ static void bdrv_drain_invoke(BlockDriverState *bs, bool begin, bool recursive)
>  
>  static bool bdrv_drain_recurse(BlockDriverState *bs)
>  {
> -    BdrvChild *child, *tmp;
> +    BdrvChild *child;
>      bool waited;
> +    struct BDSToDrain {
> +        BlockDriverState *bs;
> +        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();
>  
>      /* Wait for drained requests to finish */
>      waited = BDRV_POLL_WHILE(bs, atomic_read(&bs->in_flight) > 0);
>  
> -    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);
> +    /* Draining children may result in other children being removed from this
> +     * parent 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;
>          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.
> +             * 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(bs);
> +            bdrv_ref(bs2d->bs);
>          }
> -        waited |= bdrv_drain_recurse(bs);
> +
> +        QLIST_INSERT_HEAD(&bs_list, bs2d, next);
> +    }
> +
> +    while (!QLIST_EMPTY(&bs_list)) {
> +        struct BDSToDrain *bs2d = QLIST_FIRST(&bs_list);
> +        QLIST_REMOVE(bs2d, next);
> +
> +        assert(bs2d->bs->refcnt > 0);
> +
> +        waited |= bdrv_drain_recurse(bs2d->bs);
>          if (in_main_loop) {
> -            bdrv_unref(bs);
> +            bdrv_unref(bs2d->bs);
>          }
> +
> +        g_free(bs2d);
>      }
>  
>      return waited;
> -- 
> 2.14.3
> 
> 

Reviewed-by: Jeff Cody <jcody@redhat.com>