nbd: factor out a helper to get nbd_config without holding 'config_lock'

There are no functional changes, just to make code cleaner and prepare
to fix null-ptr-dereference while accessing 'nbd->config'.
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20231116162316.1740402-3-linan666@huaweicloud.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -395,6 +395,14 @@ static u32 req_to_nbd_cmd_type(struct request *req)
 	}
 }
 
+static struct nbd_config *nbd_get_config_unlocked(struct nbd_device *nbd)
+{
+	if (refcount_inc_not_zero(&nbd->config_refs))
+		return nbd->config;
+
+	return NULL;
+}
+
 static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req)
 {
 	struct nbd_cmd *cmd = blk_mq_rq_to_pdu(req);
@@ -409,13 +417,13 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req)
 		return BLK_EH_DONE;
 	}
 
-	if (!refcount_inc_not_zero(&nbd->config_refs)) {
+	config = nbd_get_config_unlocked(nbd);
+	if (!config) {
 		cmd->status = BLK_STS_TIMEOUT;
 		__clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
 		mutex_unlock(&cmd->lock);
 		goto done;
 	}
-	config = nbd->config;
 
 	if (config->num_connections > 1 ||
 	    (config->num_connections == 1 && nbd->tag_set.timeout)) {
@@ -977,12 +985,12 @@ static int nbd_handle_cmd(struct nbd_cmd *cmd, int index)
 	struct nbd_sock *nsock;
 	int ret;
 
-	if (!refcount_inc_not_zero(&nbd->config_refs)) {
+	config = nbd_get_config_unlocked(nbd);
+	if (!config) {
 		dev_err_ratelimited(disk_to_dev(nbd->disk),
 				    "Socks array is empty\n");
 		return -EINVAL;
 	}
-	config = nbd->config;
 
 	if (index >= config->num_connections) {
 		dev_err_ratelimited(disk_to_dev(nbd->disk),
@@ -1560,6 +1568,7 @@ static int nbd_alloc_and_init_config(struct nbd_device *nbd)
 static int nbd_open(struct gendisk *disk, blk_mode_t mode)
 {
 	struct nbd_device *nbd;
+	struct nbd_config *config;
 	int ret = 0;
 
 	mutex_lock(&nbd_index_mutex);
@@ -1572,7 +1581,9 @@ static int nbd_open(struct gendisk *disk, blk_mode_t mode)
 		ret = -ENXIO;
 		goto out;
 	}
-	if (!refcount_inc_not_zero(&nbd->config_refs)) {
+
+	config = nbd_get_config_unlocked(nbd);
+	if (!config) {
 		mutex_lock(&nbd->config_lock);
 		if (refcount_inc_not_zero(&nbd->config_refs)) {
 			mutex_unlock(&nbd->config_lock);
@@ -1588,7 +1599,7 @@ static int nbd_open(struct gendisk *disk, blk_mode_t mode)
 		mutex_unlock(&nbd->config_lock);
 		if (max_part)
 			set_bit(GD_NEED_PART_SCAN, &disk->state);
-	} else if (nbd_disconnected(nbd->config)) {
+	} else if (nbd_disconnected(config)) {
 		if (max_part)
 			set_bit(GD_NEED_PART_SCAN, &disk->state);
 	}
@@ -2205,7 +2216,8 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
 	}
 	mutex_unlock(&nbd_index_mutex);
 
-	if (!refcount_inc_not_zero(&nbd->config_refs)) {
+	config = nbd_get_config_unlocked(nbd);
+	if (!config) {
 		dev_err(nbd_to_dev(nbd),
 			"not configured, cannot reconfigure\n");
 		nbd_put(nbd);
@@ -2213,7 +2225,6 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
 	}
 
 	mutex_lock(&nbd->config_lock);
-	config = nbd->config;
 	if (!test_bit(NBD_RT_BOUND, &config->runtime_flags) ||
 	    !nbd->pid) {
 		dev_err(nbd_to_dev(nbd),