[lwip-devel] [bug #29271] Application can't re-use pbufs

[Simon Goldschmidt]

URL:
  <http://savannah.nongnu.org/bugs/?29271>

                 Summary: Application can't re-use pbufs
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: goldsimon
            Submitted on: Sa 20 Mär 2010 10:40:29 GMT
                Category: pbufs
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: CVS Head

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Details:

This has been mentioned multiple times, especially regarding ARP-queueing and
zero-copy netif drivers:

Currently, we have no clear documentation whether an application may reuse a
pbuf passed to udp_send/raw_send. I had the impression it was allowed (which
is the reason ARP-queueing has to copy even single-PBUF_RAM-pbufs when
queueing), but that isn't really correct, as the pbuf's payload is change when
udp_send returns (i.e. from the UDP payload to the ethernet header). After
all, ARP-queueing behaves just like a really slow zero-copy-MAC: it has a
reference on the pbuf until it is sent.

If we wanted to allow this we would:
a) have to limit reusing to p->ref == 1 (if > 1, the pbuf is still queued
somewhere)
b) somehow set back the payload pointer to the original status, which is kind
of hard when the pbuf is queued for sending (cannot set the payload pointer
back while returning from the various send layers since the pbuf might still
be queued).

To sum it up, I think we need to update the documentation so that an
application is not allowed to reuse pbufs passed to send functions. This also
allows easier implementation of TX-zero-copy netif drivers.




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