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

[Markus Armbruster]

Juan Quintela <address@hidden> writes:

> Markus Armbruster <address@hidden> wrote:
>> Juan Quintela <address@hidden> 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?

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.