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, 14 insertions, 3 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index d5cd3131c57a..d5bbdcfd0dab 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -4347,18 +4347,28 @@ static void cfq_exit_queue(struct elevator_queue *e)
         kfree(cfqd);
 }
 
-static int cfq_init_queue(struct request_queue *q)
+static int cfq_init_queue(struct request_queue *q, struct elevator_type *e)
 {
         struct cfq_data *cfqd;
         struct blkcg_gq *blkg __maybe_unused;
         int i, ret;
+        struct elevator_queue *eq;
+
+        eq = elevator_alloc(q, e);
+        if (!eq)
+                return -ENOMEM;
 
         cfqd = kmalloc_node(sizeof(*cfqd), GFP_KERNEL | __GFP_ZERO, q->node);
-        if (!cfqd)
+        if (!cfqd) {
+                kobject_put(&eq->kobj);
                 return -ENOMEM;
+        }
+        eq->elevator_data = cfqd;
 
         cfqd->queue = q;
-        q->elevator->elevator_data = cfqd;
+        spin_lock_irq(q->queue_lock);
+        q->elevator = eq;
+        spin_unlock_irq(q->queue_lock);
 
         /* Init root service tree */
         cfqd->grp_service_tree = CFQ_RB_ROOT;
@@ -4433,6 +4443,7 @@ static int cfq_init_queue(struct request_queue *q)
 
 out_free:
         kfree(cfqd);
+        kobject_put(&eq->kobj);
         return ret;
 }