elevator: Fix a race in elevator switching

There's a race between elevator switching and normal io operation.
    Because the allocation of struct elevator_queue and struct elevator_data
    don't in a atomic operation.So there are have chance to use NULL
    ->elevator_data.
    For example:
        Thread A:                               Thread B
        blk_queu_bio                            elevator_switch
        spin_lock_irq(q->queue_block)           elevator_alloc
        elv_merge                               elevator_init_fn

    Because call elevator_alloc, it can't hold queue_lock and the
    ->elevator_data is NULL.So at the same time, threadA call elv_merge and
    nedd some info of elevator_data.So the crash happened.

    Move the elevator_alloc into func elevator_init_fn, it make the
    operations in a atomic operation.

    Using the follow method can easy reproduce this bug
    1:dd if=/dev/sdb of=/dev/null
    2:while true;do echo noop > scheduler;echo deadline > scheduler;done

    The test method also use this method.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 13 insertions, 3 deletions

diff --git a/block/deadline-iosched.c b/block/deadline-iosched.c
index ba19a3afab79..20614a332362 100644
--- a/block/deadline-iosched.c
+++ b/block/deadline-iosched.c
@@ -337,13 +337,21 @@ static void deadline_exit_queue(struct elevator_queue *e)
 /*
  * initialize elevator private data (deadline_data).
  */
-static int deadline_init_queue(struct request_queue *q)
+static int deadline_init_queue(struct request_queue *q, struct elevator_type *e)
 {
         struct deadline_data *dd;
+        struct elevator_queue *eq;
+
+        eq = elevator_alloc(q, e);
+        if (!eq)
+                return -ENOMEM;
 
         dd = kmalloc_node(sizeof(*dd), GFP_KERNEL | __GFP_ZERO, q->node);
-        if (!dd)
+        if (!dd) {
+                kobject_put(&eq->kobj);
                 return -ENOMEM;
+        }
+        eq->elevator_data = dd;
 
         INIT_LIST_HEAD(&dd->fifo_list[READ]);
         INIT_LIST_HEAD(&dd->fifo_list[WRITE]);
@@ -355,7 +363,9 @@ static int deadline_init_queue(struct request_queue *q)
         dd->front_merges = 1;
         dd->fifo_batch = fifo_batch;
 
-        q->elevator->elevator_data = dd;
+        spin_lock_irq(q->queue_lock);
+        q->elevator = eq;
+        spin_unlock_irq(q->queue_lock);
         return 0;
 }