Subject: NET_DMA: where do we ever call dma_skb_copy_datagram_iovec() with NULL pinned_list?

From: Al Viro <[email-masked]>
Date: Sat, 21 Jul 2007 04:48:29 +0100

AFAICS, all callers of dma_skb_copy_datagram_iovec()
are either
	* recursive for fragments, pass pinned_list unchanged or
	* called from tcp, with pinned_list coming from
tp->ucopy.pinned_list and only when tp->ucopy.dma_chan is non-NULL.

Now, all non-NULL assignments to ->dma_chan have the same form:
	if (!tp->ucopy.dma_chan && tp->ucopy.pinned_list)
		tp->ucopy.dma_chan = get_softnet_dma();
IOW, if ->ucopy.pinned_list stays NULL, ->ucopy.dma_chan will do the same.

Moreover, any place that resets ->ucopy.pinned_list will also reset
->ucopy.dma_chan.

IOW, we can't ever get non-NULL tp->ucopy.dma_chan while tp->ucopy.pinned_list
is NULL.  So how can we ever get to the dma_memcpy_to_kernel_iovec()?