Re: [RFC PATCH v2 08/25] block: introduce assert_bdrv_graph_writable

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/10/21 16:31, Emanuele Giuseppe Esposito wrote:
> We want to be sure that the functions that write the child and
> parent list of a bs are either under BQL or drain.
> If this guarantee holds, then we can read the list also in the I/O APIs.
> 
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
>   block.c                                | 5 +++++
>   block/io.c                             | 5 +++++
>   include/block/block_int-global-state.h | 8 +++++++-
>   3 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/block.c b/block.c
> index b912f517a4..7d1eb847a4 100644
> --- a/block.c
> +++ b/block.c
> @@ -2711,12 +2711,14 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
>           if (child->klass->detach) {
>               child->klass->detach(child);
>           }
> +        assert_bdrv_graph_writable(old_bs);
>           QLIST_REMOVE(child, next_parent);
>       }
>   
>       child->bs = new_bs;
>   
>       if (new_bs) {
> +        assert_bdrv_graph_writable(new_bs);
>           QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
>   
>           /*
> @@ -2917,6 +2919,7 @@ static int bdrv_attach_child_noperm(BlockDriverState *parent_bs,
>           return ret;
>       }
>   
> +    assert_bdrv_graph_writable(parent_bs);
>       QLIST_INSERT_HEAD(&parent_bs->children, *child, next);
>       /*
>        * child is removed in bdrv_attach_child_common_abort(), so don't care to
> @@ -3117,6 +3120,7 @@ static void bdrv_unset_inherits_from(BlockDriverState *root, BdrvChild *child,
>   void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child)
>   {
>       g_assert(qemu_in_main_thread());
> +    assert_bdrv_graph_writable(parent);
>       if (child == NULL) {
>           return;
>       }
> @@ -4878,6 +4882,7 @@ static void bdrv_remove_filter_or_cow_child_abort(void *opaque)
>       BdrvRemoveFilterOrCowChild *s = opaque;
>       BlockDriverState *parent_bs = s->child->opaque;
>   
> +    assert_bdrv_graph_writable(parent_bs);
>       QLIST_INSERT_HEAD(&parent_bs->children, s->child, next);
>       if (s->is_backing) {
>           parent_bs->backing = s->child;
> diff --git a/block/io.c b/block/io.c
> index 21dcc5d962..d184183b07 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -739,6 +739,11 @@ void bdrv_drain_all(void)
>       bdrv_drain_all_end();
>   }
>   
> +void assert_bdrv_graph_writable(BlockDriverState *bs)
> +{
> +    g_assert(qatomic_read(&bs->quiesce_counter) > 0 || qemu_in_main_thread());
> +}

Hmm, wait - I think this should be an "&&", not an OR?

Paolo

> +
>   /**
>    * Remove an active request from the tracked requests list
>    *
> diff --git a/include/block/block_int-global-state.h b/include/block/block_int-global-state.h
> index aad549cb85..a53ab146fc 100644
> --- a/include/block/block_int-global-state.h
> +++ b/include/block/block_int-global-state.h
> @@ -343,4 +343,10 @@ void bdrv_remove_aio_context_notifier(BlockDriverState *bs,
>    */
>   void bdrv_drain_all_end_quiesce(BlockDriverState *bs);
>   
> -#endif /* BLOCK_INT_GLOBAL_STATE*/
> +/**
> + * Make sure that the function is either running under
> + * drain, or under BQL.
> + */
> +void assert_bdrv_graph_writable(BlockDriverState *bs);
> +
> +#endif /* BLOCK_INT_GLOBAL_STATE */
>