Re: [PATCH v3 33/33] block/nbd: drop connection_co

[Eric Blake <eblake@redhat.com>]

On Fri, Apr 16, 2021 at 11:09:11AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> OK, that's a big rewrite of the logic.
> 
> Pre-patch we have an always running coroutine - connection_co. It does
> reply receiving and reconnecting. And it leads to a lot of difficult
> and unobvious code around drained sections and context switch. We also
> abuse bs->in_flight counter which is increased for connection_co and
> temporary decreased in points where we want to allow drained section to
> begin. One of these place is in another file: in nbd_read_eof() in
> nbd/client.c.
> 
> We also cancel reconnect and requests waiting for reconnect on drained
> begin which is not correct.
> 
> Let's finally drop this always running coroutine and go another way:
> 
> 1. reconnect_attempt() goes to nbd_co_send_request and called under
>    send_mutex
> 
> 2. We do receive headers in request coroutine. But we also should
>    dispatch replies for another pending requests. So,
>    nbd_connection_entry() is turned into nbd_receive_replies(), which
>    does reply dispatching until it receive another request headers, and
>    returns when it receive the requested header.
> 
> 3. All old staff around drained sections and context switch is dropped.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/nbd.c  | 376 ++++++++++++++++-----------------------------------
>  nbd/client.c |   2 -
>  2 files changed, 119 insertions(+), 259 deletions(-)
> 

> -static coroutine_fn void nbd_connection_entry(void *opaque)
> +static coroutine_fn void nbd_receive_replies(BDRVNBDState *s, uint64_t handle)
>  {
> -    BDRVNBDState *s = opaque;
>      uint64_t i;
>      int ret = 0;
>      Error *local_err = NULL;
>  
> -    while (qatomic_load_acquire(&s->state) != NBD_CLIENT_QUIT) {
> -        /*
> -         * The NBD client can only really be considered idle when it has
> -         * yielded from qio_channel_readv_all_eof(), waiting for data. This is
> -         * the point where the additional scheduled coroutine entry happens
> -         * after nbd_client_attach_aio_context().
> -         *
> -         * Therefore we keep an additional in_flight reference all the time and
> -         * only drop it temporarily here.
> -         */
> +    i = HANDLE_TO_INDEX(s, handle);
> +    if (s->receive_co) {
> +        assert(s->receive_co != qemu_coroutine_self());
>  
> -        if (nbd_client_connecting(s)) {
> -            nbd_co_reconnect_loop(s);
> -        }
> +        /* Another request coroutine is receiving now */
> +        s->requests[i].receiving = true;
> +        qemu_coroutine_yield();
> +        assert(!s->requests[i].receiving);
>  
> -        if (!nbd_client_connected(s)) {
> -            continue;
> +        if (s->receive_co != qemu_coroutine_self()) {
> +            /*
> +             * We are either failed or done, caller uses nbd_client_connected()
> +             * to distinguish.
> +             */
> +            return;
>          }
> +    }
> +
> +    assert(s->receive_co == 0 || s->receive_co == qemu_coroutine_self());

s/0/NULL/ here

> +    s->receive_co = qemu_coroutine_self();
>  
> +    while (nbd_client_connected(s)) {
>          assert(s->reply.handle == 0);
>          ret = nbd_receive_reply(s->bs, s->ioc, &s->reply, &local_err);
>  
> @@ -522,8 +380,21 @@ static coroutine_fn void nbd_connection_entry(void *opaque)
>              local_err = NULL;
>          }
>          if (ret <= 0) {
> -            nbd_channel_error(s, ret ? ret : -EIO);
> -            continue;
> +            ret = ret ? ret : -EIO;
> +            nbd_channel_error(s, ret);
> +            goto out;
> +        }
> +
> +        if (!nbd_client_connected(s)) {
> +            ret = -EIO;
> +            goto out;
> +        }
> +
> +        i = HANDLE_TO_INDEX(s, s->reply.handle);
> +
> +        if (s->reply.handle == handle) {
> +            ret = 0;
> +            goto out;
>          }
>  
>          /*

I know your followup said there is more work to do before v4, but I
look forward to seeing it.

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