Re: [Qemu-devel] [PATCH 1/2] block: let blk_add/remove_aio_context_notifier() tolerate BDS changes

[Eric Blake]

On 03/06/2018 02:48 PM, Stefan Hajnoczi wrote:
> Commit 2019ba0a0197 ("block: Add AioContextNotifier functions to BB")
> added blk_add/remove_aio_context_notifier() and implemented them by
> passing through the bdrv_*() equivalent.
>
> This doesn't work across bdrv_append(), which detaches child->bs and
> re-attaches it to a new BlockDriverState.  When
> blk_remove_aio_context_notifier() is called we will access the new BDS
> instead of the one where the notifier was added!
>
> > From the point of view of the blk_*() API user, changes to the root BDS
> should be transparent.
>
> This patch maintains a list of AioContext notifiers in BlockBackend and
> adds/removes them from the BlockDriverState as needed.
>
> Reported-by: Stefano Panella <address@hidden>
> Cc: Max Reitz <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>   block/block-backend.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++
>   block/trace-events    |  2 ++
>   2 files changed, 65 insertions(+)
>
> diff --git a/block/block-backend.c b/block/block-backend.c
> index 94ffbb6a60..aa27698820 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -31,6 +31,13 @@
>   
>   static AioContext *blk_aiocb_get_aio_context(BlockAIOCB *acb);
>   
> +typedef struct BlockBackendAioNotifier {
> +    void (*attached_aio_context)(AioContext *new_context, void *opaque);
> +    void (*detach_aio_context)(void *opaque);

Why the difference in tense (past 'attached' vs. present 'detach')?

> @@ -1827,12 +1877,25 @@ void blk_remove_aio_context_notifier(BlockBackend *blk,
>                                        void (*detach_aio_context)(void *),
>                                        void *opaque)
>   {
> +    BlockBackendAioNotifier *notifier;
>       BlockDriverState *bs = blk_bs(blk);
>   
>       if (bs) {
>           bdrv_remove_aio_context_notifier(bs, attached_aio_context,
>                                            detach_aio_context, opaque);
>       }
> +
> +    QLIST_FOREACH(notifier, &blk->aio_notifiers, list) {
> +        if (notifier->attached_aio_context == attached_aio_context &&
> +            notifier->detach_aio_context == detach_aio_context &&
> +            notifier->opaque == opaque) {
> +            QLIST_REMOVE(notifier, list);

Don't you need to use QLIST_FOREACH_SAFE if you are going to modify the 
list during traversal?

Otherwise makes sense to me.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org