[ovs-dev] [RFC 1/4] netlink: Support for memory mapped Netlink sockets

[Thomas Graf]

On 05/23/14 at 08:51am, Ben Pfaff wrote:
> On Fri, May 23, 2014 at 09:03:54AM +0100, Thomas Graf wrote:
> > > Using nl_sock_send_linear() can cause messages to be reordered.  Do we
> > > need to wait for the tx ring to empty before calling it?
> > 
> > Are you referring to parallel usage of _send_linear() and
> > _send_mmap() on the same socket? The ring does support carrying
> > a do-a-linear-read event which allows parallel use while preserving
> > order but the caller may not decide to do so on his own. Going from
> > maped to unmapped on a live socket would be a nice feature but would
> > require kernel support first.
> 
> If we have a series of back-to-back nl_sock_send_mmap() calls, and one
> or a few of them are too big for the frame size, then nothing currently
> assures that the kernel will receive them in the order sent.  I am not
> sure that it matters, but that is what I was trying to point out.

OK, I understand now. Sending order for a single thread is guaranteed
because both mmaped and unmapped messages are represented by sendmsg()
syscalls (unlike in the receive scenario). The ring is checked for
messages if a sendmsg() syscall is performed without payload on a
socket with a ring setup.

I'll add this this to the nl_sock_send_mmap() function documentation.