blk-cgroup: improve error unwinding in blkg_alloc

Unwind only the previous initialization steps that happened in blkg_alloc
using goto based unwinding.  This avoids the need for the !queue special
case in blkg_free and thus ensures that any blkg seens outside of
blkg_alloc is always fully constructed.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20230203150400.3199230-4-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -128,22 +128,16 @@ static void blkg_free_workfn(struct work_struct *work)
 	 * blkcg_mutex is used to synchronize blkg_free_workfn() and
 	 * blkcg_deactivate_policy().
 	 */
-	if (q)
-		mutex_lock(&q->blkcg_mutex);
-
+	mutex_lock(&q->blkcg_mutex);
 	for (i = 0; i < BLKCG_MAX_POLS; i++)
 		if (blkg->pd[i])
 			blkcg_policy[i]->pd_free_fn(blkg->pd[i]);
-
 	if (blkg->parent)
 		blkg_put(blkg->parent);
+	list_del_init(&blkg->q_node);
+	mutex_unlock(&q->blkcg_mutex);
 
-	if (q) {
-		list_del_init(&blkg->q_node);
-		mutex_unlock(&q->blkcg_mutex);
-		blk_put_queue(q);
-	}
-
+	blk_put_queue(q);
 	free_percpu(blkg->iostat_cpu);
 	percpu_ref_exit(&blkg->refcnt);
 	kfree(blkg);
@@ -265,16 +259,13 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
 	blkg = kzalloc_node(sizeof(*blkg), gfp_mask, disk->queue->node);
 	if (!blkg)
 		return NULL;
-
 	if (percpu_ref_init(&blkg->refcnt, blkg_release, 0, gfp_mask))
-		goto err_free;
-
+		goto out_free_blkg;
 	blkg->iostat_cpu = alloc_percpu_gfp(struct blkg_iostat_set, gfp_mask);
 	if (!blkg->iostat_cpu)
-		goto err_free;
-
+		goto out_exit_refcnt;
 	if (!blk_get_queue(disk->queue))
-		goto err_free;
+		goto out_free_iostat;
 
 	blkg->q = disk->queue;
 	INIT_LIST_HEAD(&blkg->q_node);
@@ -299,8 +290,7 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
 		/* alloc per-policy data and attach it to blkg */
 		pd = pol->pd_alloc_fn(gfp_mask, disk->queue, blkcg);
 		if (!pd)
-			goto err_free;
-
+			goto out_free_pds;
 		blkg->pd[i] = pd;
 		pd->blkg = blkg;
 		pd->plid = i;
@@ -309,8 +299,17 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
 
 	return blkg;
 
-err_free:
-	blkg_free(blkg);
+out_free_pds:
+	while (--i >= 0)
+		if (blkg->pd[i])
+			blkcg_policy[i]->pd_free_fn(blkg->pd[i]);
+	blk_put_queue(disk->queue);
+out_free_iostat:
+	free_percpu(blkg->iostat_cpu);
+out_exit_refcnt:
+	percpu_ref_exit(&blkg->refcnt);
+out_free_blkg:
+	kfree(blkg);
 	return NULL;
 }