Commit 59ea33a6 authored Jul 27, 2012 by Jiri Kosina Committed by David S. Miller Jul 27, 2012

tcp: perform DMA to userspace only if there is a task waiting for it



Back in 2006, commit 1a2449a8 ("[I/OAT]: TCP recv offload to I/OAT")
added support for receive offloading to IOAT dma engine if available.

The code in tcp_rcv_established() tries to perform early DMA copy if
applicable. It however does so without checking whether the userspace
task is actually expecting the data in the buffer.

This is not a problem under normal circumstances, but there is a corner
case where this doesn't work -- and that's when MSG_TRUNC flag to
recvmsg() is used.

If the IOAT dma engine is not used, the code properly checks whether
there is a valid ucopy.task and the socket is owned by userspace, but
misses the check in the dmaengine case.

This problem can be observed in real trivially -- for example 'tbench' is a
good reproducer, as it makes a heavy use of MSG_TRUNC. On systems utilizing
IOAT, you will soon find tbench waiting indefinitely in sk_wait_data(), as they
have been already early-copied in tcp_rcv_established() using dma engine.

This patch introduces the same check we are performing in the simple
iovec copy case to the IOAT case as well. It fixes the indefinite
recvmsg(MSG_TRUNC) hangs.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 60810307

net/ipv4/tcp_input.c

+3 −1

Original line number	Original line	Diff line number	Diff line
	@@ -5475,7 +5475,9 @@ int tcp_rcv_established(struct sock sk, struct sk_buff skb,
	if (tp->copied_seq == tp->rcv_nxt &&		if (tp->copied_seq == tp->rcv_nxt &&
	len - tcp_header_len <= tp->ucopy.len) {		len - tcp_header_len <= tp->ucopy.len) {
	#ifdef CONFIG_NET_DMA		#ifdef CONFIG_NET_DMA
	if (tcp_dma_try_early_copy(sk, skb, tcp_header_len)) {		if (tp->ucopy.task == current &&
			sock_owned_by_user(sk) &&
			tcp_dma_try_early_copy(sk, skb, tcp_header_len)) {
	copied_early = 1;		copied_early = 1;
	eaten = 1;		eaten = 1;
	}		}