blkcg: Drop unnecessary RCU read [un]locks from blkg_conf_prep/finish()

Now that all RCU flavors have been combined either holding a spin lock,
disabling irq or disabling preemption implies RCU read lock, so there's no
need to use rcu_read_[un]lock() explicitly while holding queue_lock. This
shouldn't cause any behavior changes.

v2: Description updated. Leave __acquires/release on queue_lock alone.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230413000649.115785-2-tj@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -699,12 +699,12 @@ struct block_device *blkcg_conf_open_bdev(char **inputp)
  *
  * Parse per-blkg config update from @input and initialize @ctx with the
  * result.  @ctx->blkg points to the blkg to be updated and @ctx->body the
- * part of @input following MAJ:MIN.  This function returns with RCU read
- * lock and queue lock held and must be paired with blkg_conf_finish().
+ * part of @input following MAJ:MIN.  This function returns with queue lock
+ * held and must be paired with blkg_conf_finish().
  */
 int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		   char *input, struct blkg_conf_ctx *ctx)
-	__acquires(rcu) __acquires(&bdev->bd_queue->queue_lock)
+	__acquires(&bdev->bd_queue->queue_lock)
 {
 	struct block_device *bdev;
 	struct gendisk *disk;
@@ -726,7 +726,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 	if (ret)
 		goto fail;
 
-	rcu_read_lock();
 	spin_lock_irq(&q->queue_lock);
 
 	if (!blkcg_policy_enabled(q, pol)) {
@@ -755,7 +754,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 
 		/* Drop locks to do new blkg allocation with GFP_KERNEL. */
 		spin_unlock_irq(&q->queue_lock);
-		rcu_read_unlock();
 
 		new_blkg = blkg_alloc(pos, disk, GFP_KERNEL);
 		if (unlikely(!new_blkg)) {
@@ -769,7 +767,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 			goto fail_exit_queue;
 		}
 
-		rcu_read_lock();
 		spin_lock_irq(&q->queue_lock);
 
 		if (!blkcg_policy_enabled(q, pol)) {
@@ -805,7 +802,6 @@ fail_preloaded:
 	radix_tree_preload_end();
 fail_unlock:
 	spin_unlock_irq(&q->queue_lock);
-	rcu_read_unlock();
 fail_exit_queue:
 	blk_queue_exit(q);
 fail:
@@ -832,10 +828,9 @@ EXPORT_SYMBOL_GPL(blkg_conf_prep);
  * with blkg_conf_prep().
  */
 void blkg_conf_finish(struct blkg_conf_ctx *ctx)
-	__releases(&ctx->bdev->bd_queue->queue_lock) __releases(rcu)
+	__releases(&ctx->bdev->bd_queue->queue_lock)
 {
 	spin_unlock_irq(&bdev_get_queue(ctx->bdev)->queue_lock);
-	rcu_read_unlock();
 	blkdev_put_no_open(ctx->bdev);
 }
 EXPORT_SYMBOL_GPL(blkg_conf_finish);