The proc connector messages include a sequence number, allowing userspace
programs to detect lost messages.  However, performing this detection is
currently more difficult than necessary, since netlink messages can be
delivered to the application out-of-order.  To fix this, leave pre-emption
disabled during cn_netlink_send(), and use GFP_NOWAIT.

The following was written as a test case.  Building the kernel w/ make -j32
proved a reliable way to generate out-of-order cn_proc messages.

int
main(int argc, char *argv[])
{
	static uint32_t last_seq[CPU_SETSIZE], seq;
	int cpu, fd;
	struct sockaddr_nl sa;
	struct __attribute__((aligned(NLMSG_ALIGNTO))) {
		struct nlmsghdr nl_hdr;
		struct __attribute__((__packed__)) {
			struct cn_msg cn_msg;
			struct proc_event cn_proc;
		};
	} rmsg;
	struct __attribute__((aligned(NLMSG_ALIGNTO))) {
		struct nlmsghdr nl_hdr;
		struct __attribute__((__packed__)) {
			struct cn_msg cn_msg;
			enum proc_cn_mcast_op cn_mcast;
		};
	} smsg;

	fd = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
	if (fd < 0) {
		perror("socket");
	}

	sa.nl_family = AF_NETLINK;
	sa.nl_groups = CN_IDX_PROC;
	sa.nl_pid = getpid();
	if (bind(fd, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
		perror("bind");
	}

	memset(&smsg, 0, sizeof(smsg));
	smsg.nl_hdr.nlmsg_len = sizeof(smsg);
	smsg.nl_hdr.nlmsg_pid = getpid();
	smsg.nl_hdr.nlmsg_type = NLMSG_DONE;
	smsg.cn_msg.id.idx = CN_IDX_PROC;
	smsg.cn_msg.id.val = CN_VAL_PROC;
	smsg.cn_msg.len = sizeof(enum proc_cn_mcast_op);
	smsg.cn_mcast = PROC_CN_MCAST_LISTEN;
	if (send(fd, &smsg, sizeof(smsg), 0) != sizeof(smsg)) {
		perror("send");
	}

	while (recv(fd, &rmsg, sizeof(rmsg), 0) == sizeof(rmsg)) {
		cpu = rmsg.cn_proc.cpu;
		if (cpu < 0) {
			continue;
		}
		seq = rmsg.cn_msg.seq;
		if ((last_seq[cpu] != 0) && (seq != last_seq[cpu] + 1)) {
			printf("out-of-order seq=%d on cpu=%d\n", seq, cpu);
		}
		last_seq[cpu] = seq;
	}

	/* NOTREACHED */

	perror("recv");

	return -1;
}

Signed-off-by: Aaron Campbell <aaron@monkey.org>
Signed-off-by: David S. Miller <davem@davemloft.net>