block: add function call to switch the IO scheduler from a driver

Currently drivers must do an elevator_exit() + elevator_init()
to switch IO schedulers. There are a few problems with this:

- Since commit 1abec4fdbb142e3ccb6ce99832fae42129134a96,
  elevator_init() requires a zeroed out q->elevator
  pointer. The two existing in-kernel users don't do that.

- It will only work at initialization time, since using the
  above two-staged construct does not properly quisce the queue.

So add elevator_change() which takes care of this, and convert
the elv_iosched_store() sysfs interface to use this helper as well.

Reported-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Reported-by: Kevin Vigor <kevin@vigor.nu>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

2 files changed, 32 insertions, 13 deletions

diff --git a/block/elevator.c b/block/elevator.c
index ec585c9554d3..205b09a5bd9e 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -1009,18 +1009,19 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 {
         struct elevator_queue *old_elevator, *e;
         void *data;
+        int err;
 
         /*
          * Allocate new elevator
          */
         e = elevator_alloc(q, new_e);
         if (!e)
-                return 0;
+                return -ENOMEM;
 
         data = elevator_init_queue(q, e);
         if (!data) {
                 kobject_put(&e->kobj);
-                return 0;
+                return -ENOMEM;
         }
 
         /*
@@ -1043,7 +1044,8 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 
         __elv_unregister_queue(old_elevator);
 
-        if (elv_register_queue(q))
+        err = elv_register_queue(q);
+        if (err)
                 goto fail_register;
 
         /*
@@ -1056,7 +1058,7 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 
         blk_add_trace_msg(q, "elv switch: %s", e->elevator_type->elevator_name);
 
-        return 1;
+        return 0;
 
 fail_register:
         /*
@@ -1071,17 +1073,19 @@ fail_register:
         queue_flag_clear(QUEUE_FLAG_ELVSWITCH, q);
         spin_unlock_irq(q->queue_lock);
 
-        return 0;
+        return err;
 }
 
-ssize_t elv_iosched_store(struct request_queue *q, const char *name,
-                          size_t count)
+/*
+ * Switch this queue to the given IO scheduler.
+ */
+int elevator_change(struct request_queue *q, const char *name)
 {
         char elevator_name[ELV_NAME_MAX];
         struct elevator_type *e;
 
         if (!q->elevator)
-                return count;
+                return -ENXIO;
 
         strlcpy(elevator_name, name, sizeof(elevator_name));
         e = elevator_get(strstrip(elevator_name));
@@ -1092,13 +1096,27 @@ ssize_t elv_iosched_store(struct request_queue *q, const char *name,
 
         if (!strcmp(elevator_name, q->elevator->elevator_type->elevator_name)) {
                 elevator_put(e);
-                return count;
+                return 0;
         }
 
-        if (!elevator_switch(q, e))
-                printk(KERN_ERR "elevator: switch to %s failed\n",
-                                                        elevator_name);
-        return count;
+        return elevator_switch(q, e);
+}
+EXPORT_SYMBOL(elevator_change);
+
+ssize_t elv_iosched_store(struct request_queue *q, const char *name,
+                          size_t count)
+{
+        int ret;
+
+        if (!q->elevator)
+                return count;
+
+        ret = elevator_change(q, name);
+        if (!ret)
+                return count;
+
+        printk(KERN_ERR "elevator: switch to %s failed\n", name);
+        return ret;
 }
 
 ssize_t elv_iosched_show(struct request_queue *q, char *name)
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index 2c958f4fce1e..926b50322a46 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -136,6 +136,7 @@ extern ssize_t elv_iosched_store(struct request_queue *, const char *, size_t);
 
 extern int elevator_init(struct request_queue *, char *);
 extern void elevator_exit(struct elevator_queue *);
+extern int elevator_change(struct request_queue *, const char *);
 extern int elv_rq_merge_ok(struct request *, struct bio *);
 
 /*