tcp_recvmesg hang under NET_DMA

["Steven J. Magnani" <steve@digidescorp.com>]

Hi,

I've run into a case where using NET_DMA seems to cause a PUT to a
modified ftpd to hang. AFAICT this is because the NET_DMA design queues
up lots of memcpy operations, but doesn't issue or wait for them (and
thus free the associated skbs) until it is time for tcp_recvmesg() to
return. In a case where userland requests a large read (as is the case
in my modified ftpd), packet transfer can grind to a halt because so
many packets are buffered up and there is no event that causes them to
be freed.

I can submit a patch that would issue pending memcpys and clean up
completed ones around tcp_recvmsg's call to sk_wait_data(), but first I
wanted to understand the design better. Were there specific reasons for
deferring the async operations so long?

Thanks,
------------------------------------------------------------------------
 Steven J. Magnani               "I claim this network for MARS!
 www.digidescorp.com              Earthling, return my space modulator!"

 #include <standard.disclaimer>