static void vmbus_process_offer()

in channel_mgmt.c [575:684]

• 41 lines of code
• 8 McCabe index (conditional complexity)

static void vmbus_process_offer(struct vmbus_channel *newchannel)
{
	struct vmbus_channel *channel;
	struct workqueue_struct *wq;
	bool fnew = true;

	/*
	 * Synchronize vmbus_process_offer() and CPU hotplugging:
	 *
	 * CPU1				CPU2
	 *
	 * [vmbus_process_offer()]	[Hot removal of the CPU]
	 *
	 * CPU_READ_LOCK		CPUS_WRITE_LOCK
	 * LOAD cpu_online_mask		SEARCH chn_list
	 * STORE target_cpu		LOAD target_cpu
	 * INSERT chn_list		STORE cpu_online_mask
	 * CPUS_READ_UNLOCK		CPUS_WRITE_UNLOCK
	 *
	 * Forbids: CPU1's LOAD from *not* seing CPU2's STORE &&
	 *              CPU2's SEARCH from *not* seeing CPU1's INSERT
	 *
	 * Forbids: CPU2's SEARCH from seeing CPU1's INSERT &&
	 *              CPU2's LOAD from *not* seing CPU1's STORE
	 */
	cpus_read_lock();

	/*
	 * Serializes the modifications of the chn_list list as well as
	 * the accesses to next_numa_node_id in init_vp_index().
	 */
	mutex_lock(&vmbus_connection.channel_mutex);

	list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
		if (guid_equal(&channel->offermsg.offer.if_type,
			       &newchannel->offermsg.offer.if_type) &&
		    guid_equal(&channel->offermsg.offer.if_instance,
			       &newchannel->offermsg.offer.if_instance)) {
			fnew = false;
			newchannel->primary_channel = channel;
			break;
		}
	}

	init_vp_index(newchannel);

	/* Remember the channels that should be cleaned up upon suspend. */
	if (is_hvsock_channel(newchannel) || is_sub_channel(newchannel))
		atomic_inc(&vmbus_connection.nr_chan_close_on_suspend);

	/*
	 * Now that we have acquired the channel_mutex,
	 * we can release the potentially racing rescind thread.
	 */
	atomic_dec(&vmbus_connection.offer_in_progress);

	if (fnew) {
		list_add_tail(&newchannel->listentry,
			      &vmbus_connection.chn_list);
	} else {
		/*
		 * Check to see if this is a valid sub-channel.
		 */
		if (newchannel->offermsg.offer.sub_channel_index == 0) {
			mutex_unlock(&vmbus_connection.channel_mutex);
			/*
			 * Don't call free_channel(), because newchannel->kobj
			 * is not initialized yet.
			 */
			kfree(newchannel);
			WARN_ON_ONCE(1);
			return;
		}
		/*
		 * Process the sub-channel.
		 */
		list_add_tail(&newchannel->sc_list, &channel->sc_list);
	}

	vmbus_channel_map_relid(newchannel);

	mutex_unlock(&vmbus_connection.channel_mutex);
	cpus_read_unlock();

	/*
	 * vmbus_process_offer() mustn't call channel->sc_creation_callback()
	 * directly for sub-channels, because sc_creation_callback() ->
	 * vmbus_open() may never get the host's response to the
	 * OPEN_CHANNEL message (the host may rescind a channel at any time,
	 * e.g. in the case of hot removing a NIC), and vmbus_onoffer_rescind()
	 * may not wake up the vmbus_open() as it's blocked due to a non-zero
	 * vmbus_connection.offer_in_progress, and finally we have a deadlock.
	 *
	 * The above is also true for primary channels, if the related device
	 * drivers use sync probing mode by default.
	 *
	 * And, usually the handling of primary channels and sub-channels can
	 * depend on each other, so we should offload them to different
	 * workqueues to avoid possible deadlock, e.g. in sync-probing mode,
	 * NIC1's netvsc_subchan_work() can race with NIC2's netvsc_probe() ->
	 * rtnl_lock(), and causes deadlock: the former gets the rtnl_lock
	 * and waits for all the sub-channels to appear, but the latter
	 * can't get the rtnl_lock and this blocks the handling of
	 * sub-channels.
	 */
	INIT_WORK(&newchannel->add_channel_work, vmbus_add_channel_work);
	wq = fnew ? vmbus_connection.handle_primary_chan_wq :
		    vmbus_connection.handle_sub_chan_wq;
	queue_work(wq, &newchannel->add_channel_work);
}