Re: [PATCH v5 09/31] block: introduce assert_bdrv_graph_writable

[Hanna Reitz]

On 24.11.21 07:43, Emanuele Giuseppe Esposito wrote:
> We want to be sure that the functions that write the child and
> parent list of a bs are under BQL and drain.
>
> BQL prevents from concurrent writings from the GS API, while
> drains protect from I/O.
>
> TODO: drains are missing in some functions using this assert.
> Therefore a proper assertion will fail. Because adding drains
> requires additional discussions, they will be added in future
> series.
>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
>   include/block/block_int-global-state.h | 10 +++++++++-
>   block.c                                |  4 ++++
>   block/io.c                             | 11 +++++++++++
>   3 files changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/include/block/block_int-global-state.h 
> b/include/block/block_int-global-state.h
> index a1b7d0579d..fa96e8b449 100644
> --- a/include/block/block_int-global-state.h
> +++ b/include/block/block_int-global-state.h
> @@ -312,4 +312,12 @@ void bdrv_remove_aio_context_notifier(BlockDriverState *bs,
>    */
>   void bdrv_drain_all_end_quiesce(BlockDriverState *bs);
>   
> -#endif /* BLOCK_INT_GLOBAL_STATE*/

This looks like it should be squashed into patch 7, sorry I missed this 
in v4...

(Rest of this patch looks good to me, for the record – and while I’m at 
it, for patches I didn’t reply to so far, I planned to send an R-b 
later.  But then there’s things like patches 2/3 looking good to me, but 
it turned out in my review for patch 4 that bdrv_lock_medium() is used 
in an I/O path, so I can’t really send an R-b now anymore...)

Hanna

> +/**
> + * Make sure that the function is running under both drain and BQL.
> + * The latter protects from concurrent writings
> + * from the GS API, while the former prevents concurrent reads
> + * from I/O.
> + */
> +void assert_bdrv_graph_writable(BlockDriverState *bs);
> +
> +#endif /* BLOCK_INT_GLOBAL_STATE */
> diff --git a/block.c b/block.c
> index 198ec636ff..522a273140 100644
> --- a/block.c
> +++ b/block.c
> @@ -1416,6 +1416,7 @@ static void bdrv_child_cb_attach(BdrvChild *child)
>   {
>       BlockDriverState *bs = child->opaque;
>   
> +    assert_bdrv_graph_writable(bs);
>       QLIST_INSERT_HEAD(&bs->children, child, next);
>   
>       if (child->role & BDRV_CHILD_COW) {
> @@ -1435,6 +1436,7 @@ static void bdrv_child_cb_detach(BdrvChild *child)
>   
>       bdrv_unapply_subtree_drain(child, bs);
>   
> +    assert_bdrv_graph_writable(bs);
>       QLIST_REMOVE(child, next);
>   }
>   
> @@ -2818,6 +2820,7 @@ static void bdrv_replace_child_noperm(BdrvChild **childp,
>           if (child->klass->detach) {
>               child->klass->detach(child);
>           }
> +        assert_bdrv_graph_writable(old_bs);
>           QLIST_REMOVE(child, next_parent);
>       }
>   
> @@ -2827,6 +2830,7 @@ static void bdrv_replace_child_noperm(BdrvChild **childp,
>       }
>   
>       if (new_bs) {
> +        assert_bdrv_graph_writable(new_bs);
>           QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
>   
>           /*
> diff --git a/block/io.c b/block/io.c
> index cb095deeec..3be08cad29 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -734,6 +734,17 @@ void bdrv_drain_all(void)
>       bdrv_drain_all_end();
>   }
>   
> +void assert_bdrv_graph_writable(BlockDriverState *bs)
> +{
> +    /*
> +     * TODO: this function is incomplete. Because the users of this
> +     * assert lack the necessary drains, check only for BQL.
> +     * Once the necessary drains are added,
> +     * assert also for qatomic_read(&bs->quiesce_counter) > 0
> +     */
> +    assert(qemu_in_main_thread());
> +}
> +
>   /**
>    * Remove an active request from the tracked requests list
>    *