Re: [RFC] block/nbd: Move s->ioc on AioContext change

[Hanna Reitz]

On 01.02.22 12:18, Vladimir Sementsov-Ogievskiy wrote:
> 28.01.2022 18:51, Hanna Reitz wrote:
> > s->ioc must always be attached to the NBD node's AioContext.  If that
> > context changes, s->ioc must be attached to the new context.
> >
> > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1990835
> > Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> > ---
> > This is an RFC because I believe there are some other things in the NBD
> > block driver that need attention on an AioContext change, too. Namely,
> > there are two timers (reconnect_delay_timer and open_timer) that are
> > also attached to the node's AioContext, and I'm afraid they need to be
> > handled, too.  Probably pause them on detach, and resume them on attach,
> > but I'm not sure, which is why I'm posting this as an RFC to get some
> > comments from that from someone who knows this code better than me. :)
> >
> > (Also, in a real v1, of course I'd want to add a regression test.)
> > ---
> >   block/nbd.c | 28 ++++++++++++++++++++++++++++
> >   1 file changed, 28 insertions(+)
> >
> > diff --git a/block/nbd.c b/block/nbd.c
> > index 63dbfa807d..119a774c04 100644
> > --- a/block/nbd.c
> > +++ b/block/nbd.c
> > @@ -2036,6 +2036,25 @@ static void 
> > nbd_cancel_in_flight(BlockDriverState *bs)
> >       nbd_co_establish_connection_cancel(s->conn);
> >   }
> >   +static void nbd_attach_aio_context(BlockDriverState *bs,
> > +                                   AioContext *new_context)
> > +{
> > +    BDRVNBDState *s = bs->opaque;
> > +
> > +    if (s->ioc) {
> > +        qio_channel_attach_aio_context(s->ioc, new_context);
> > +    }
> > +}
> > +
> > +static void nbd_detach_aio_context(BlockDriverState *bs)
> > +{
> > +    BDRVNBDState *s = bs->opaque;
> > +
> > +    if (s->ioc) {
> > +        qio_channel_detach_aio_context(s->ioc);
> > +    }
> > +}
> > +
> >   static BlockDriver bdrv_nbd = {
> >       .format_name                = "nbd",
> >       .protocol_name              = "nbd",
> > @@ -2059,6 +2078,9 @@ static BlockDriver bdrv_nbd = {
> >       .bdrv_dirname               = nbd_dirname,
> >       .strong_runtime_opts        = nbd_strong_runtime_opts,
> >       .bdrv_cancel_in_flight      = nbd_cancel_in_flight,
> > +
> > +    .bdrv_attach_aio_context    = nbd_attach_aio_context,
> > +    .bdrv_detach_aio_context    = nbd_detach_aio_context,
> >   };
> >     static BlockDriver bdrv_nbd_tcp = {
> > @@ -2084,6 +2106,9 @@ static BlockDriver bdrv_nbd_tcp = {
> >       .bdrv_dirname               = nbd_dirname,
> >       .strong_runtime_opts        = nbd_strong_runtime_opts,
> >       .bdrv_cancel_in_flight      = nbd_cancel_in_flight,
> > +
> > +    .bdrv_attach_aio_context    = nbd_attach_aio_context,
> > +    .bdrv_detach_aio_context    = nbd_detach_aio_context,
> >   };
> >     static BlockDriver bdrv_nbd_unix = {
> > @@ -2109,6 +2134,9 @@ static BlockDriver bdrv_nbd_unix = {
> >       .bdrv_dirname               = nbd_dirname,
> >       .strong_runtime_opts        = nbd_strong_runtime_opts,
> >       .bdrv_cancel_in_flight      = nbd_cancel_in_flight,
> > +
> > +    .bdrv_attach_aio_context    = nbd_attach_aio_context,
> > +    .bdrv_detach_aio_context    = nbd_detach_aio_context,
> >   };
> >     static void bdrv_nbd_init(void)
>
>
> Hmm. I was so happy to remove these handlers together with 
> connection-coroutine :) . But you are right, seems I've removed too 
> much :(.
>
>
> open_timer exists only during bdrv_open() handler, so, I hope on 
> attach/detach it should not exist.

That’s… kind of surprising.  It’s good for me here, but as far as I can 
see it means that all of qemu blocks until the connection succeeds, 
right?  That doesn’t seem quite ideal...

Anyway, good for me. O:)

> reconnect_delay_timer should exist only during IO request: it's 
> created during request if we don't have a connection. And request will 
> not finish until timer elapsed or connection established (timer should 
> be removed in this case too). So, again, when attaching / detaching 
> the context we should be in a drained sections, so no in-flight 
> requests and no reconnect_delay_timer.

Got it.  FWIW, other block drivers rely on this, too (e.g. null-aio with 
latency-ns set creates a timer in every I/O request and settles the 
request once the timer expires).

> So, I think assertions that both timer pointers are NULL should be 
> enough in attach / detach handlers.

Great!  I’ll cook up v1.