Re: [Qemu-devel] [PATCH 07/18] migration: make writes blocking

[Juan Quintela <quintela@redhat.com>]

Markus Armbruster <armbru@redhat.com> wrote:
> Juan Quintela <quintela@redhat.com> writes:
>
>> Markus Armbruster <armbru@redhat.com> wrote:
>>> Juan Quintela <quintela@redhat.com> writes:
>>>
>>>> Move all the writes to the migration_thread, and make writings
>>>> blocking.  Notice that are still using the iothread for everything
>>>> that we do.
>>> [...]
>>>> diff --git a/qemu-sockets.c b/qemu-sockets.c
>>>> index cfed9c5..61b6e95 100644
>>>> --- a/qemu-sockets.c
>>>> +++ b/qemu-sockets.c
>>>> @@ -276,9 +276,6 @@ static int inet_connect_addr(struct addrinfo *addr, bool *in_progress,
>>>>          return -1;
>>>>      }
>>>>      qemu_setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
>>>> -    if (connect_state != NULL) {
>>>> -        socket_set_nonblock(sock);
>>>> -    }
>>>>      /* connect to peer */
>>>>      do {
>>>>          rc = 0;
>>>> @@ -732,7 +729,6 @@ int unix_connect_opts(QemuOpts *opts, Error **errp,
>>>>          connect_state = g_malloc0(sizeof(*connect_state));
>>>>          connect_state->callback = callback;
>>>>          connect_state->opaque = opaque;
>>>> -        socket_set_nonblock(sock);
>>>>      }
>>>>
>>>>      memset(&un, 0, sizeof(un));
>>>
>>> Doesn't this break inet_nonblocking_connect() and
>>> unix_nonblocking_connect()?
>>>
>>> In your cover letter, you wrote:
>>>
>>>     Note: Writes has become blocking, and I have to change the "remove"
>>>     the feature now in qemu-sockets.c.  Checked that migration was the
>>>     only user of that feature.  If new users appear, they just need to add
>>>     the socket_set_nonblock() by hand.
>>>
>>> Yes, migration-{tcp,unix} are their only users, but if they want a
>>> blocking socket now, why not use inet_connect() and unix_connect()?
>>>
>>> New users can't "just add socket_set_nonblock()".  They'd have to add it
>>> right where you deleted it: between qemu_socket() and connect().  Else
>>> the connect() is blocking.
>>
>> Grrr.
>>
>> So, is there any way to make a connection that is non-blocking, but then
>> writes are blocking?
>
> Which operations on the migration socket do you need to block, and which
> ones do you need not to block?

connect: not blocking (done on the iothread)
writes: blocking, done in the migration thread.

I think thet socket_set_block() that paolo says is the right solution.

>
> If connect() should block, use inet_connect() / unix_connect().  The
> returned socket will be blocking.  You can then switch to non-blocking
> mode (and possibly back) at appropriate times.
>
> If connect() should not block, use inet_nonblocking_connect() and so
> forth.

Thanks, Juan.