Removing queue length check in ip_output (was Re: buf_ring in HEAD is racy)

[Adrian Chadd]

On 18 December 2013 14:40, Garrett Wollman
<wollman at hergotha.csail.mit.edu> wrote:
> In article
> <CAJ-VmomyPq_2K-MFhb7vt6MM7RBbmn7yaTzUXb7+N7TbW1RmHQ at mail.gmail.com>,
> adrian at freebsd.org writes:
>
>>.. it's totally bogus in an if_transmit / SMP world. There's no
>>locking and there's no guarantee that there will be headroom in the
>>queue between this point and the later call to the if_output() method.
>
> The theory is that when transmitting fragments, it's desirable to have
> all-or-nothing transmit behavior with respect to the original packet.
> Transmitting incomplete fragmented packets wastes resources on the
> local CPU, in the network, and on the remote system, so you should not
> do so when it is possible to avoid it.

Right, but this API definitely can't guarantee this as it presently
stands anyway.

> Ideally, there would be an if_reserve(ifp, N) interface that hands you
> a token that you can then pass back N times to if_transmit().  But
> it's not clear to me how common or necessary this is, except perhaps
> on very busy authoritative DNS servers.  (Are there any other
> applications for fragmented UDP these days?)  So it might not be worth
> the effort.

Ideally we'd pass a list of fragments to the next layer up and such
and said layers would treat that list as a "send all or nothing" unit.

I have the same issues in the wifi stack with 802.11 fragments.



-a