NBD: allow nbd to be used locally

This patch allows Network Block Device to be mounted locally (nbd-client to
nbd-server over 127.0.0.1).

It creates a kthread to avoid the deadlock described in NBD tools
documentation.  So, if nbd-client hangs waiting for pages, the kblockd thread
can continue its work and free pages.

I have tested the patch to verify that it avoids the hang that always occurs
when writing to a localhost nbd connection.  I have also tested to verify that
no performance degradation results from the additional thread and queue.

Patch originally from Laurent Vivier.

Signed-off-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 98 insertions, 50 deletions

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 60cc54368b66..8e33de6bea33 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -29,6 +29,7 @@
 #include <linux/kernel.h>
 #include <net/sock.h>
 #include <linux/net.h>
+#include <linux/kthread.h>
 
 #include <asm/uaccess.h>
 #include <asm/system.h>
@@ -441,6 +442,85 @@ static void nbd_clear_que(struct nbd_device *lo)
 }
 
 
+static void nbd_handle_req(struct nbd_device *lo, struct request *req)
+{
+        if (!blk_fs_request(req))
+                goto error_out;
+
+        nbd_cmd(req) = NBD_CMD_READ;
+        if (rq_data_dir(req) == WRITE) {
+                nbd_cmd(req) = NBD_CMD_WRITE;
+                if (lo->flags & NBD_READ_ONLY) {
+                        printk(KERN_ERR "%s: Write on read-only\n",
+                                        lo->disk->disk_name);
+                        goto error_out;
+                }
+        }
+
+        req->errors = 0;
+
+        mutex_lock(&lo->tx_lock);
+        if (unlikely(!lo->sock)) {
+                mutex_unlock(&lo->tx_lock);
+                printk(KERN_ERR "%s: Attempted send on closed socket\n",
+                       lo->disk->disk_name);
+                req->errors++;
+                nbd_end_request(req);
+                return;
+        }
+
+        lo->active_req = req;
+
+        if (nbd_send_req(lo, req) != 0) {
+                printk(KERN_ERR "%s: Request send failed\n",
+                                lo->disk->disk_name);
+                req->errors++;
+                nbd_end_request(req);
+        } else {
+                spin_lock(&lo->queue_lock);
+                list_add(&req->queuelist, &lo->queue_head);
+                spin_unlock(&lo->queue_lock);
+        }
+
+        lo->active_req = NULL;
+        mutex_unlock(&lo->tx_lock);
+        wake_up_all(&lo->active_wq);
+
+        return;
+
+error_out:
+        req->errors++;
+        nbd_end_request(req);
+}
+
+static int nbd_thread(void *data)
+{
+        struct nbd_device *lo = data;
+        struct request *req;
+
+        set_user_nice(current, -20);
+        while (!kthread_should_stop() || !list_empty(&lo->waiting_queue)) {
+                /* wait for something to do */
+                wait_event_interruptible(lo->waiting_wq,
+                                         kthread_should_stop() ||
+                                         !list_empty(&lo->waiting_queue));
+
+                /* extract request */
+                if (list_empty(&lo->waiting_queue))
+                        continue;
+
+                spin_lock_irq(&lo->queue_lock);
+                req = list_entry(lo->waiting_queue.next, struct request,
+                                 queuelist);
+                list_del_init(&req->queuelist);
+                spin_unlock_irq(&lo->queue_lock);
+
+                /* handle request */
+                nbd_handle_req(lo, req);
+        }
+        return 0;
+}
+
 /*
  * We always wait for result of write, for now. It would be nice to make it optional
  * in future
@@ -456,65 +536,23 @@ static void do_nbd_request(struct request_queue * q)
                 struct nbd_device *lo;
 
                 blkdev_dequeue_request(req);
+
+                spin_unlock_irq(q->queue_lock);
+
                 dprintk(DBG_BLKDEV, "%s: request %p: dequeued (flags=%x)\n",
                                 req->rq_disk->disk_name, req, req->cmd_type);
 
-                if (!blk_fs_request(req))
-                        goto error_out;
-
                 lo = req->rq_disk->private_data;
 
                 BUG_ON(lo->magic != LO_MAGIC);
 
-                nbd_cmd(req) = NBD_CMD_READ;
-                if (rq_data_dir(req) == WRITE) {
-                        nbd_cmd(req) = NBD_CMD_WRITE;
-                        if (lo->flags & NBD_READ_ONLY) {
-                                printk(KERN_ERR "%s: Write on read-only\n",
-                                                lo->disk->disk_name);
-                                goto error_out;
-                        }
-                }
-
-                req->errors = 0;
-                spin_unlock_irq(q->queue_lock);
-
-                mutex_lock(&lo->tx_lock);
-                if (unlikely(!lo->sock)) {
-                        mutex_unlock(&lo->tx_lock);
-                        printk(KERN_ERR "%s: Attempted send on closed socket\n",
-                               lo->disk->disk_name);
-                        req->errors++;
-                        nbd_end_request(req);
-                        spin_lock_irq(q->queue_lock);
-                        continue;
-                }
+                spin_lock_irq(&lo->queue_lock);
+                list_add_tail(&req->queuelist, &lo->waiting_queue);
+                spin_unlock_irq(&lo->queue_lock);
 
-                lo->active_req = req;
-
-                if (nbd_send_req(lo, req) != 0) {
-                        printk(KERN_ERR "%s: Request send failed\n",
-                                        lo->disk->disk_name);
-                        req->errors++;
-                        nbd_end_request(req);
-                } else {
-                        spin_lock(&lo->queue_lock);
-                        list_add(&req->queuelist, &lo->queue_head);
-                        spin_unlock(&lo->queue_lock);
-                }
-
-                lo->active_req = NULL;
-                mutex_unlock(&lo->tx_lock);
-                wake_up_all(&lo->active_wq);
+                wake_up(&lo->waiting_wq);
 
                 spin_lock_irq(q->queue_lock);
-                continue;
-
-error_out:
-                req->errors++;
-                spin_unlock(q->queue_lock);
-                nbd_end_request(req);
-                spin_lock(q->queue_lock);
         }
 }
 
@@ -524,6 +562,7 @@ static int nbd_ioctl(struct inode *inode, struct file *file,
         struct nbd_device *lo = inode->i_bdev->bd_disk->private_data;
         int error;
         struct request sreq ;
+        struct task_struct *thread;
 
         if (!capable(CAP_SYS_ADMIN))
                 return -EPERM;
@@ -606,7 +645,12 @@ static int nbd_ioctl(struct inode *inode, struct file *file,
         case NBD_DO_IT:
                 if (!lo->file)
                         return -EINVAL;
+                thread = kthread_create(nbd_thread, lo, lo->disk->disk_name);
+                if (IS_ERR(thread))
+                        return PTR_ERR(thread);
+                wake_up_process(thread);
                 error = nbd_do_it(lo);
+                kthread_stop(thread);
                 if (error)
                         return error;
                 sock_shutdown(lo, 1);
@@ -695,10 +739,12 @@ static int __init nbd_init(void)
                 nbd_dev[i].file = NULL;
                 nbd_dev[i].magic = LO_MAGIC;
                 nbd_dev[i].flags = 0;
+                INIT_LIST_HEAD(&nbd_dev[i].waiting_queue);
                 spin_lock_init(&nbd_dev[i].queue_lock);
                 INIT_LIST_HEAD(&nbd_dev[i].queue_head);
                 mutex_init(&nbd_dev[i].tx_lock);
                 init_waitqueue_head(&nbd_dev[i].active_wq);
+                init_waitqueue_head(&nbd_dev[i].waiting_wq);
                 nbd_dev[i].blksize = 1024;
                 nbd_dev[i].bytesize = 0;
                 disk->major = NBD_MAJOR;
diff --git a/include/linux/nbd.h b/include/linux/nbd.h
index 986572081e19..69075517c511 100644
--- a/include/linux/nbd.h
+++ b/include/linux/nbd.h
@@ -56,9 +56,11 @@ struct nbd_device {
         int magic;
 
         spinlock_t queue_lock;
-        struct list_head queue_head;/* Requests are added here...       */
+        struct list_head queue_head;    /* Requests waiting result */
         struct request *active_req;
         wait_queue_head_t active_wq;
+        struct list_head waiting_queue; /* Requests to be sent */
+        wait_queue_head_t waiting_wq;
 
         struct mutex tx_lock;
         struct gendisk *disk;