Re: [PATCH 8/8] [I/OAT] TCP recv offload to I/OAT

Chris Leech [off-list ref] wrote:

+#ifdef CONFIG_NET_DMA
 +	tp->ucopy.dma_chan = NULL;
 +	if ((len > sysctl_tcp_dma_copybreak) && !(flags & MSG_PEEK) && !sysctl_tcp_low_latency && __get_cpu_var(softnet_data.net_dma))
 +		dma_lock_iovec_pages(msg->msg_iov, len, &tp->ucopy.locked_list);
 +#endif

The __get_cpu_var() here will run smp_processor_id() from preemptible
context.  You'll get a big warning if the correct debug options are set.

The reason for this is that preemption could cause this code to hop between
CPUs.

Please always test code with all debug options enabled and with full kernel
preemption.