Move the blkg_conf_open_bdev() call out of blkg_conf_prep() to make it
possible to add lock context annotations to blkg_conf_prep(). Change an
if-statement in blkg_conf_open_bdev() into a WARN_ON_ONCE() call. Export
blkg_conf_open_bdev() because it is called by the BFQ I/O scheduler and
the BFQ I/O scheduler may be built as a kernel module.

Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/bfq-cgroup.c |  4 ++++
 block/blk-cgroup.c | 16 ++++++----------
 block/blk-iocost.c |  4 ++++
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index ac83b0668764..38396df9dce7 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -1051,6 +1051,10 @@ static ssize_t bfq_io_set_device_weight(struct kernfs_open_file *of,
 
 	blkg_conf_init(&ctx, buf);
 
+	ret = blkg_conf_open_bdev(&ctx);
+	if (ret)
+		goto out;
+
 	ret = blkg_conf_prep(blkcg, &blkcg_policy_bfq, &ctx);
 	if (ret)
 		goto out;
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 2d7b18eb7291..8cfdbe9fffdc 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -771,10 +771,7 @@ EXPORT_SYMBOL_GPL(blkg_conf_init);
  * @ctx->input and get and store the matching bdev in @ctx->bdev. @ctx->body is
  * set to point past the device node prefix.
  *
- * This function may be called multiple times on @ctx and the extra calls become
- * NOOPs. blkg_conf_prep() implicitly calls this function. Use this function
- * explicitly if bdev access is needed without resolving the blkcg / policy part
- * of @ctx->input. Returns -errno on error.
+ * Returns: -errno on error.
  */
 int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx)
 {
@@ -783,8 +780,7 @@ int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx)
 	struct block_device *bdev;
 	int key_len;
 
-	if (ctx->bdev)
-		return 0;
+	WARN_ON_ONCE(ctx->bdev);
 
 	if (sscanf(input, "%u:%u%n", &major, &minor, &key_len) != 2)
 		return -EINVAL;
@@ -813,6 +809,8 @@ int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx)
 	ctx->bdev = bdev;
 	return 0;
 }
+EXPORT_SYMBOL_GPL(blkg_conf_open_bdev);
+
 /*
  * Similar to blkg_conf_open_bdev, but additionally freezes the queue,
  * ensures the correct locking order between freeze queue and q->rq_qos_mutex.
@@ -857,7 +855,7 @@ unsigned long __must_check blkg_conf_open_bdev_frozen(struct blkg_conf_ctx *ctx)
  * following MAJ:MIN, @ctx->bdev points to the target block device and
  * @ctx->blkg to the blkg being configured.
  *
- * blkg_conf_open_bdev() may be called on @ctx beforehand. On success, this
+ * blkg_conf_open_bdev() must be called on @ctx beforehand. On success, this
  * function returns with queue lock held and must be followed by
  * blkg_conf_exit().
  */
@@ -870,9 +868,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 	struct blkcg_gq *blkg;
 	int ret;
 
-	ret = blkg_conf_open_bdev(ctx);
-	if (ret)
-		return ret;
+	WARN_ON_ONCE(!ctx->bdev);
 
 	disk = ctx->bdev->bd_disk;
 	q = disk->queue;
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index d145db61e5c3..7c90241a1dbd 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -3129,6 +3129,10 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
 
 	blkg_conf_init(&ctx, buf);
 
+	ret = blkg_conf_open_bdev(&ctx);
+	if (ret)
+		goto err;
+
 	ret = blkg_conf_prep(blkcg, &blkcg_policy_iocost, &ctx);
 	if (ret)
 		goto err;