[PATCH] uml: fix ubd deadlock on SMP

From: BlaisorBlade <blaisorblade_spam@yahoo.it>, Chris Wright <chrisw@osdl.org>

Avoid deadlocking onto the request lock in the UBD driver, i.e.  don't lock
the queue spinlock when called from the request function.

In detail:

Rename ubd_finish() to __ubd_finish() and remove ubd_io_lock from it.  Add
wrapper, ubd_finish(), which grabs lock before calling __ubd_finish().  Update
do_ubd_request to use the lock free __ubd_finish() to avoid deadlock.  Also,
apparently prepare_request is called with ubd_io_lock held, so remove locks
there.
Signed-off-by: Chris Wright <chrisw@osdl.org>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/drivers/ubd_kern.c

@@ -396,14 +396,13 @@ int thread_fd = -1;
  */
 int intr_count = 0;
 
-static void ubd_finish(struct request *req, int error)
+/* call ubd_finish if you need to serialize */
+static void __ubd_finish(struct request *req, int error)
 {
 	int nsect;
 
 	if(error){
- 		spin_lock(&ubd_io_lock);
 		end_request(req, 0);
- 		spin_unlock(&ubd_io_lock);
 		return;
 	}
 	nsect = req->current_nr_sectors;
@@ -412,11 +411,17 @@ static void ubd_finish(struct request *req, int error)
 	req->errors = 0;
 	req->nr_sectors -= nsect;
 	req->current_nr_sectors = 0;
-	spin_lock(&ubd_io_lock);
 	end_request(req, 1);
+}
+
+static inline void ubd_finish(struct request *req, int error)
+{
+ 	spin_lock(&ubd_io_lock);
+	__ubd_finish(req, error);
 	spin_unlock(&ubd_io_lock);
 }
 
+/* Called without ubd_io_lock held */
 static void ubd_handler(void)
 {
 	struct io_thread_req req;
@@ -965,6 +970,7 @@ static int prepare_mmap_request(struct ubd *dev, int fd, __u64 offset,
 	return(0);
 }
 
+/* Called with ubd_io_lock held */
 static int prepare_request(struct request *req, struct io_thread_req *io_req)
 {
 	struct gendisk *disk = req->rq_disk;
@@ -977,9 +983,7 @@ static int prepare_request(struct request *req, struct io_thread_req *io_req)
 	if((rq_data_dir(req) == WRITE) && !dev->openflags.w){
 		printk("Write attempted on readonly ubd device %s\n", 
 		       disk->disk_name);
- 		spin_lock(&ubd_io_lock);
 		end_request(req, 0);
- 		spin_unlock(&ubd_io_lock);
 		return(1);
 	}
 
@@ -1029,6 +1033,7 @@ static int prepare_request(struct request *req, struct io_thread_req *io_req)
 	return(0);
 }
 
+/* Called with ubd_io_lock held */
 static void do_ubd_request(request_queue_t *q)
 {
 	struct io_thread_req io_req;
@@ -1040,7 +1045,7 @@ static void do_ubd_request(request_queue_t *q)
 			err = prepare_request(req, &io_req);
 			if(!err){
 				do_io(&io_req);
-				ubd_finish(req, io_req.error);
+				__ubd_finish(req, io_req.error);
 			}
 		}
 	}