blk-mq: convert to serialize updating nr_requests with update_nr_hwq_lock

request_queue->nr_requests can be changed by:

a) switch elevator by updating nr_hw_queues
b) switch elevator by elevator sysfs attribute
c) configue queue sysfs attribute nr_requests

Current lock order is:

1) update_nr_hwq_lock, case a,b
2) freeze_queue
3) elevator_lock, case a,b,c

And update nr_requests is seriablized by elevator_lock() already,
however, in the case c, we'll have to allocate new sched_tags if
nr_requests grow, and do this with elevator_lock held and queue
freezed has the risk of deadlock.

Hence use update_nr_hwq_lock instead, make it possible to allocate
memory if tags grow, meanwhile also prevent nr_requests to be changed
concurrently.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index f99519f7a8201bd8c6beb93066ecff589861f333..ff66d1b471699555ff1112b83ae0a707d448c7da 100644 (file)
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -68,13 +68,17 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
        int ret, err;
        unsigned int memflags;
        struct request_queue *q = disk->queue;
+       struct blk_mq_tag_set *set = q->tag_set;
 
        ret = queue_var_store(&nr, page, count);
        if (ret < 0)
                return ret;
 
-       memflags = blk_mq_freeze_queue(q);
-       mutex_lock(&q->elevator_lock);
+       /*
+        * Serialize updating nr_requests with concurrent queue_requests_store()
+        * and switching elevator.
+        */
+       down_write(&set->update_nr_hwq_lock);
 
        if (nr == q->nr_requests)
                goto unlock;
@@ -82,20 +86,31 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
        if (nr < BLKDEV_MIN_RQ)
                nr = BLKDEV_MIN_RQ;
 
-       if (nr <= q->tag_set->reserved_tags ||
+       /*
+        * Switching elevator is protected by update_nr_hwq_lock:
+        *  - read lock is held from elevator sysfs attribute;
+        *  - write lock is held from updating nr_hw_queues;
+        * Hence it's safe to access q->elevator here with write lock held.
+        */
+       if (nr <= set->reserved_tags ||
            (q->elevator && nr > MAX_SCHED_RQ) ||
-           (!q->elevator && nr > q->tag_set->queue_depth)) {
+           (!q->elevator && nr > set->queue_depth)) {
                ret = -EINVAL;
                goto unlock;
        }
 
+       memflags = blk_mq_freeze_queue(q);
+       mutex_lock(&q->elevator_lock);
+
        err = blk_mq_update_nr_requests(disk->queue, nr);
        if (err)
                ret = err;
 
-unlock:
        mutex_unlock(&q->elevator_lock);
        blk_mq_unfreeze_queue(q, memflags);
+
+unlock:
+       up_write(&set->update_nr_hwq_lock);
        return ret;
 }