NBD: allow hung network I/O to be cancelled

Allow NBD I/O to be cancelled when a network outage occurs.  Previously, I/O
would just hang, and if enough I/O was hung in nbd, the system (at least
user-level) would completely hang until a TCP timeout (default, 15 minutes)
occurred.

The patch introduces a new ioctl NBD_SET_TIMEOUT that allows a transmit
timeout value (in seconds) to be specified.  Any network send that exceeds the
timeout will be cancelled and the nbd connection will be shut down.  I've
tested with various timeout values and 6 seconds seems to be a good choice for
the timeout.  If the NBD_SET_TIMEOUT ioctl is not called, you get the old (I/O
hang) behavior.
Signed-off-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/nbd.c

@@ -113,12 +113,42 @@ static void nbd_end_request(struct request *req)
 	spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
+static void sock_shutdown(struct nbd_device *lo, int lock)
+{
+	/* Forcibly shutdown the socket causing all listeners
+	 * to error
+	 *
+	 * FIXME: This code is duplicated from sys_shutdown, but
+	 * there should be a more generic interface rather than
+	 * calling socket ops directly here */
+	if (lock)
+		mutex_lock(&lo->tx_lock);
+	if (lo->sock) {
+		printk(KERN_WARNING "%s: shutting down socket\n",
+			lo->disk->disk_name);
+		lo->sock->ops->shutdown(lo->sock, SEND_SHUTDOWN|RCV_SHUTDOWN);
+		lo->sock = NULL;
+	}
+	if (lock)
+		mutex_unlock(&lo->tx_lock);
+}
+
+static void nbd_xmit_timeout(unsigned long arg)
+{
+	struct task_struct *task = (struct task_struct *)arg;
+
+	printk(KERN_WARNING "nbd: killing hung xmit (%s, pid: %d)\n",
+		task->comm, task->pid);
+	force_sig(SIGKILL, task);
+}
+
 /*
  *  Send or receive packet.
  */
-static int sock_xmit(struct socket *sock, int send, void *buf, int size,
+static int sock_xmit(struct nbd_device *lo, int send, void *buf, int size,
 		int msg_flags)
 {
+	struct socket *sock = lo->sock;
 	int result;
 	struct msghdr msg;
 	struct kvec iov;
@@ -139,9 +169,20 @@ static int sock_xmit(struct socket *sock, int send, void *buf, int size,
 		msg.msg_controllen = 0;
 		msg.msg_flags = msg_flags | MSG_NOSIGNAL;
 
-		if (send)
+		if (send) {
+			struct timer_list ti;
+
+			if (lo->xmit_timeout) {
+				init_timer(&ti);
+				ti.function = nbd_xmit_timeout;
+				ti.data = (unsigned long)current;
+				ti.expires = jiffies + lo->xmit_timeout;
+				add_timer(&ti);
+			}
 			result = kernel_sendmsg(sock, &msg, &iov, 1, size);
-		else
+			if (lo->xmit_timeout)
+				del_timer_sync(&ti);
+		} else
 			result = kernel_recvmsg(sock, &msg, &iov, 1, size, 0);
 
 		if (signal_pending(current)) {
@@ -150,6 +191,7 @@ static int sock_xmit(struct socket *sock, int send, void *buf, int size,
 				current->pid, current->comm,
 				dequeue_signal_lock(current, &current->blocked, &info));
 			result = -EINTR;
+			sock_shutdown(lo, !send);
 			break;
 		}
 
@@ -167,23 +209,22 @@ static int sock_xmit(struct socket *sock, int send, void *buf, int size,
 	return result;
 }
 
-static inline int sock_send_bvec(struct socket *sock, struct bio_vec *bvec,
+static inline int sock_send_bvec(struct nbd_device *lo, struct bio_vec *bvec,
 		int flags)
 {
 	int result;
 	void *kaddr = kmap(bvec->bv_page);
-	result = sock_xmit(sock, 1, kaddr + bvec->bv_offset, bvec->bv_len,
-			flags);
+	result = sock_xmit(lo, 1, kaddr + bvec->bv_offset, bvec->bv_len, flags);
 	kunmap(bvec->bv_page);
 	return result;
 }
 
+/* always call with the tx_lock held */
 static int nbd_send_req(struct nbd_device *lo, struct request *req)
 {
 	int result, flags;
 	struct nbd_request request;
 	unsigned long size = req->nr_sectors << 9;
-	struct socket *sock = lo->sock;
 
 	request.magic = htonl(NBD_REQUEST_MAGIC);
 	request.type = htonl(nbd_cmd(req));
@@ -196,8 +237,8 @@ static int nbd_send_req(struct nbd_device *lo, struct request *req)
 			nbdcmd_to_ascii(nbd_cmd(req)),
 			(unsigned long long)req->sector << 9,
 			req->nr_sectors << 9);
-	result = sock_xmit(sock, 1, &request, sizeof(request),
-			(nbd_cmd(req) == NBD_CMD_WRITE)? MSG_MORE: 0);
+	result = sock_xmit(lo, 1, &request, sizeof(request),
+			(nbd_cmd(req) == NBD_CMD_WRITE) ? MSG_MORE : 0);
 	if (result <= 0) {
 		printk(KERN_ERR "%s: Send control failed (result %d)\n",
 				lo->disk->disk_name, result);
@@ -217,7 +258,7 @@ static int nbd_send_req(struct nbd_device *lo, struct request *req)
 				flags = MSG_MORE;
 			dprintk(DBG_TX, "%s: request %p: sending %d bytes data\n",
 					lo->disk->disk_name, req, bvec->bv_len);
-			result = sock_send_bvec(sock, bvec, flags);
+			result = sock_send_bvec(lo, bvec, flags);
 			if (result <= 0) {
 				printk(KERN_ERR "%s: Send data failed (result %d)\n",
 						lo->disk->disk_name, result);
@@ -257,11 +298,11 @@ out:
 	return ERR_PTR(err);
 }
 
-static inline int sock_recv_bvec(struct socket *sock, struct bio_vec *bvec)
+static inline int sock_recv_bvec(struct nbd_device *lo, struct bio_vec *bvec)
 {
 	int result;
 	void *kaddr = kmap(bvec->bv_page);
-	result = sock_xmit(sock, 0, kaddr + bvec->bv_offset, bvec->bv_len,
+	result = sock_xmit(lo, 0, kaddr + bvec->bv_offset, bvec->bv_len,
 			MSG_WAITALL);
 	kunmap(bvec->bv_page);
 	return result;
@@ -273,10 +314,9 @@ static struct request *nbd_read_stat(struct nbd_device *lo)
 	int result;
 	struct nbd_reply reply;
 	struct request *req;
-	struct socket *sock = lo->sock;
 
 	reply.magic = 0;
-	result = sock_xmit(sock, 0, &reply, sizeof(reply), MSG_WAITALL);
+	result = sock_xmit(lo, 0, &reply, sizeof(reply), MSG_WAITALL);
 	if (result <= 0) {
 		printk(KERN_ERR "%s: Receive control failed (result %d)\n",
 				lo->disk->disk_name, result);
@@ -317,7 +357,7 @@ static struct request *nbd_read_stat(struct nbd_device *lo)
 		struct bio_vec *bvec;
 
 		rq_for_each_segment(bvec, req, iter) {
-			result = sock_recv_bvec(sock, bvec);
+			result = sock_recv_bvec(lo, bvec);
 			if (result <= 0) {
 				printk(KERN_ERR "%s: Receive data failed (result %d)\n",
 						lo->disk->disk_name, result);
@@ -392,6 +432,7 @@ static void nbd_clear_que(struct nbd_device *lo)
 	}
 }
 
+
 /*
  * We always wait for result of write, for now. It would be nice to make it optional
  * in future
@@ -500,7 +541,9 @@ static int nbd_ioctl(struct inode *inode, struct file *file,
 		sreq.nr_sectors = 0;
                 if (!lo->sock)
 			return -EINVAL;
+		mutex_lock(&lo->tx_lock);
                 nbd_send_req(lo, &sreq);
+		mutex_unlock(&lo->tx_lock);
                 return 0;
  
 	case NBD_CLEAR_SOCK:
@@ -544,6 +587,9 @@ static int nbd_ioctl(struct inode *inode, struct file *file,
 		set_blocksize(inode->i_bdev, lo->blksize);
 		set_capacity(lo->disk, lo->bytesize >> 9);
 		return 0;
+	case NBD_SET_TIMEOUT:
+		lo->xmit_timeout = arg * HZ;
+		return 0;
 	case NBD_SET_SIZE_BLOCKS:
 		lo->bytesize = ((u64) arg) * lo->blksize;
 		inode->i_bdev->bd_inode->i_size = lo->bytesize;
@@ -556,22 +602,7 @@ static int nbd_ioctl(struct inode *inode, struct file *file,
 		error = nbd_do_it(lo);
 		if (error)
 			return error;
-		/* on return tidy up in case we have a signal */
-		/* Forcibly shutdown the socket causing all listeners
-		 * to error
-		 *
-		 * FIXME: This code is duplicated from sys_shutdown, but
-		 * there should be a more generic interface rather than
-		 * calling socket ops directly here */
-		mutex_lock(&lo->tx_lock);
-		if (lo->sock) {
-			printk(KERN_WARNING "%s: shutting down socket\n",
-				lo->disk->disk_name);
-			lo->sock->ops->shutdown(lo->sock,
-				SEND_SHUTDOWN|RCV_SHUTDOWN);
-			lo->sock = NULL;
-		}
-		mutex_unlock(&lo->tx_lock);
+		sock_shutdown(lo, 1);
 		file = lo->file;
 		lo->file = NULL;
 		nbd_clear_que(lo);

include/linux/nbd.h

@@ -26,6 +26,7 @@
 #define NBD_PRINT_DEBUG	_IO( 0xab, 6 )
 #define NBD_SET_SIZE_BLOCKS	_IO( 0xab, 7 )
 #define NBD_DISCONNECT  _IO( 0xab, 8 )
+#define NBD_SET_TIMEOUT _IO( 0xab, 9 )
 
 enum {
 	NBD_CMD_READ = 0,
@@ -65,6 +66,7 @@ struct nbd_device {
 	int blksize;
 	u64 bytesize;
 	pid_t pid; /* pid of nbd-client, if attached */
+	int xmit_timeout;
 };
 
 #endif