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

[Eric Blake]

On 03/12/2018 06:27 AM, Stefan Hajnoczi wrote:
> On Fri, Mar 09, 2018 at 09:56:44AM -0600, Eric Blake wrote:
> > 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.

> > > +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')?
>
> The naming comes from the bdrv_add_aio_context_notifier() API:
>
>    void bdrv_add_aio_context_notifier(BlockDriverState *bs,
>          void (*attached_aio_context)(AioContext *new_context, void *opaque),
>          void (*detach_aio_context)(void *opaque), void *opaque)
>
> It's "attached" because bs->aio_context has already been assigned before
> the callback is invoked.
>
> It's "detach" because the callback is invoked before bs->aio_context is
> cleared.
>
> Not great naming and I found it weird when I looked at the code too, but
> at least this patch keeps the BlockBackend naming consistent with the
> BlockDriverState naming.

Odd, but consistent, so I can live with it.


> > > +
> > > +    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?
>
> It doesn't matter since we return right away:
>
>    g_free(notifier);
>    return;

Okay, makes sense (a safe iteration is required if you keep iterating 
after action; but if you quit, the action has no impact on subsequent 
iteration since there is no further iteration).

Reviewed-by: Eric Blake <address@hidden>

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