NBD Hangs

[Steve Whitehouse]

Hi,

I've been having some NBD hangs with the 2.4.2 kernel. It appears that
block devices which don't use plugging will not have their request
functions called when the request is queued. I propose the following
patch to ll_rw_blk.c which seems to do the trick for me. Also I have
done a small amount of tidying in nbd.c.

Btw, do you know what the deadlock problem is when plugging is enabled
on nbd ? I had a good look through the code and did some tests and there
certainly are problems with deadlocks with plugging enabled but I couldn't
quite pin down the source. I guessed it might have something to do with 
blocking in the request function.

Thanks,

Steve.

------------------------------------------------------------------------------

diff -u -r linux-2.4.2-zc1/drivers/block/ll_rw_blk.c linux/drivers/block/ll_rw_blk.c
--- linux-2.4.2-zc1/drivers/block/ll_rw_blk.c	Thu Feb 22 19:46:23 2001
+++ linux/drivers/block/ll_rw_blk.c	Sun Feb 25 11:23:42 2001
@@ -588,6 +588,9 @@
 	 * inserted at elevator_merge time
 	 */
 	list_add(&req->queue, insert_here);
+
+	if (!q->plugged && insert_here == &q->queue_head)
+		q->request_fn(q);
 }
 
 void inline blk_refill_freelist(request_queue_t *q, int rw)
diff -u -r linux-2.4.2-zc1/drivers/block/nbd.c linux/drivers/block/nbd.c
--- linux-2.4.2-zc1/drivers/block/nbd.c	Mon Oct 30 22:30:33 2000
+++ linux/drivers/block/nbd.c	Sun Feb 25 11:23:45 2001
@@ -29,7 +29,7 @@
 #include <linux/major.h>
 
 #include <linux/module.h>
-
+#include <linux/init.h>
 #include <linux/sched.h>
 #include <linux/fs.h>
 #include <linux/stat.h>
@@ -149,12 +149,13 @@
 {
 	int result;
 	struct nbd_request request;
+	unsigned long size = req->current_nr_sectors << 9;
 
 	DEBUG("NBD: sending control, ");
 	request.magic = htonl(NBD_REQUEST_MAGIC);
 	request.type = htonl(req->cmd);
 	request.from = cpu_to_be64( (u64) req->sector << 9);
-	request.len = htonl(req->current_nr_sectors << 9);
+	request.len = htonl(size);
 	memcpy(request.handle, &req, sizeof(req));
 
 	result = nbd_xmit(1, sock, (char *) &request, sizeof(request));
@@ -163,7 +164,7 @@
 
 	if (req->cmd == WRITE) {
 		DEBUG("data, ");
-		result = nbd_xmit(1, sock, req->buffer, req->current_nr_sectors << 9);
+		result = nbd_xmit(1, sock, req->buffer, size);
 		if (result <= 0)
 			FAIL("Send data failed.");
 	}
@@ -174,8 +175,11 @@
 }
 
 #define HARDFAIL( s ) { printk( KERN_ERR "NBD: " s "(result %d)\n", result ); lo->harderror = result; return NULL; }
+
+/* 
+ * NULL returned = something went wrong, inform userspace
+ */ 
 struct request *nbd_read_stat(struct nbd_device *lo)
-		/* NULL returned = something went wrong, inform userspace       */ 
 {
 	int result;
 	struct nbd_reply reply;
@@ -475,11 +479,7 @@
  *  (Just smiley confuses emacs :-)
  */
 
-#ifdef MODULE
-#define nbd_init init_module
-#endif
-
-int nbd_init(void)
+int __init nbd_init(void)
 {
 	int i;
 
@@ -493,9 +493,7 @@
 		       MAJOR_NR);
 		return -EIO;
 	}
-#ifdef MODULE
 	printk("nbd: registered device at major %d\n", MAJOR_NR);
-#endif
 	blksize_size[MAJOR_NR] = nbd_blksizes;
 	blk_size[MAJOR_NR] = nbd_sizes;
 	blk_init_queue(BLK_DEFAULT_QUEUE(MAJOR_NR), do_nbd_request);
@@ -526,8 +524,7 @@
 	return 0;
 }
 
-#ifdef MODULE
-void cleanup_module(void)
+void __exit nbd_cleanup(void)
 {
 	devfs_unregister (devfs_handle);
 	blk_cleanup_queue(BLK_DEFAULT_QUEUE(MAJOR_NR));
@@ -537,4 +534,9 @@
 	else
 		printk("nbd: module cleaned up.\n");
 }
-#endif
+
+module_init(nbd_init);
+module_exit(nbd_cleanup);
+
+MODULE_DESCRIPTION("Network Block Device");
+