bfq: Make sure bfqg for which we are queueing requests is online

Bios queued into BFQ IO scheduler can be associated with a cgroup that
was already offlined. This may then cause insertion of this bfq_group
into a service tree. But this bfq_group will get freed as soon as last
bio associated with it is completed leading to use after free issues for
service tree users. Fix the problem by making sure we always operate on
online bfq_group. If the bfq_group associated with the bio is not
online, we pick the first online parent.

CC: [email protected]
Fixes: e21b7a0 ("block, bfq: add full hierarchical scheduling and cgroups support")
Tested-by: "yukuai (C)" <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

block/bfq-cgroup.c

@@ -612,10 +612,19 @@ static void bfq_link_bfqg(struct bfq_data *bfqd, struct bfq_group *bfqg)
 struct bfq_group *bfq_bio_bfqg(struct bfq_data *bfqd, struct bio *bio)
 {
 	struct blkcg_gq *blkg = bio->bi_blkg;
+	struct bfq_group *bfqg;
 
-	if (!blkg)
-		return bfqd->root_group;
-	return blkg_to_bfqg(blkg);
+	while (blkg) {
+		bfqg = blkg_to_bfqg(blkg);
+		if (bfqg->online) {
+			bio_associate_blkg_from_css(bio, &blkg->blkcg->css);
+			return bfqg;
+		}
+		blkg = blkg->parent;
+	}
+	bio_associate_blkg_from_css(bio,
+				&bfqg_to_blkg(bfqd->root_group)->blkcg->css);
+	return bfqd->root_group;
 }
 
 /**