[PATCH 2/2] block: don't request module during elevator init

[Tejun Heo <tj@kernel.org>]

Block layer allows selecting an elevator which is built as a module to
be selected as system default via kernel param "elevator=".  This is
achieved by automatically invoking request_module() whenever a new
block device is initialized and the elevator is not available.

This led to an interesting deadlock problem involving async and module
init.  Block device probing running off an async job invokes
request_module().  While the module is being loaded, it performs
async_synchronize_full() which ends up waiting for the async job which
is already waiting for request_module() to finish, leading to
deadlock.

Invoking request_module() from deep in block device init path is
already nasty in itself.  It seems best to avoid these situations from
the beginning by moving on-demand module loading out of block init
path.

The previous patch made sure that the default elevator module is
loaded early during boot if available.  This patch removes on-demand
loading of the default elevator from elevator init path.  As the
module would have been loaded during boot, userland-visible behavior
difference should be minimal.

For more details, please refer to the following thread.

  http://thread.gmane.org/gmane.linux.kernel/1420814

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alex Riesen <raa.lkml@gmail.com>
---
 block/elevator.c |   19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

--- a/block/elevator.c
+++ b/block/elevator.c
@@ -100,14 +100,14 @@ static void elevator_put(struct elevator
 	module_put(e->elevator_owner);
 }
 
-static struct elevator_type *elevator_get(const char *name)
+static struct elevator_type *elevator_get(const char *name, bool request_module)
 {
 	struct elevator_type *e;
 
 	spin_lock(&elv_list_lock);
 
 	e = elevator_find(name);
-	if (!e) {
+	if (!e && request_module) {
 		spin_unlock(&elv_list_lock);
 		request_module("%s-iosched", name);
 		spin_lock(&elv_list_lock);
@@ -207,25 +207,30 @@ int elevator_init(struct request_queue *
 	q->boundary_rq = NULL;
 
 	if (name) {
-		e = elevator_get(name);
+		e = elevator_get(name, true);
 		if (!e)
 			return -EINVAL;
 	}
 
+	/*
+	 * Use the default elevator specified by config boot param or
+	 * config option.  Don't try to load modules as we could be running
+	 * off async and request_module() isn't allowed from async.
+	 */
 	if (!e && *chosen_elevator) {
-		e = elevator_get(chosen_elevator);
+		e = elevator_get(chosen_elevator, false);
 		if (!e)
 			printk(KERN_ERR "I/O scheduler %s not found\n",
 							chosen_elevator);
 	}
 
 	if (!e) {
-		e = elevator_get(CONFIG_DEFAULT_IOSCHED);
+		e = elevator_get(CONFIG_DEFAULT_IOSCHED, false);
 		if (!e) {
 			printk(KERN_ERR
 				"Default I/O scheduler not found. " \
 				"Using noop.\n");
-			e = elevator_get("noop");
+			e = elevator_get("noop", false);
 		}
 	}
 
@@ -967,7 +972,7 @@ int elevator_change(struct request_queue
 		return -ENXIO;
 
 	strlcpy(elevator_name, name, sizeof(elevator_name));
-	e = elevator_get(strstrip(elevator_name));
+	e = elevator_get(strstrip(elevator_name), true);
 	if (!e) {
 		printk(KERN_ERR "elevator: type %s not found\n", elevator_name);
 		return -EINVAL;