block, bfq: switch 'bfqg->ref' to use atomic refcount apis

The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
during code review, we found that bfq_pd_free() update 'bfqg->ref'
without holding the lock, which is problematic:

1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
which can concurrent with 1).

Unfortunately, 'bfqd->lock' can't be held here because 'bfqd' might already
be freed in bfq_pd_free(). Fix the problem by using atomic refcount apis.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230103084755.1256479-1-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-cgroup.c

@@ -316,14 +316,12 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
 
 static void bfqg_get(struct bfq_group *bfqg)
 {
-	bfqg->ref++;
+	refcount_inc(&bfqg->ref);
 }
 
 static void bfqg_put(struct bfq_group *bfqg)
 {
-	bfqg->ref--;
+	if (refcount_dec_and_test(&bfqg->ref))
-
-	if (bfqg->ref == 0)
 		kfree(bfqg);
 }
 
@@ -530,7 +528,7 @@ static struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, struct request_queue *q,
 	}
 
 	/* see comments in bfq_bic_update_cgroup for why refcounting */
-	bfqg_get(bfqg);
+	refcount_set(&bfqg->ref, 1);
 	return &bfqg->pd;
 }
 

block/bfq-iosched.h

@@ -928,7 +928,7 @@ struct bfq_group {
 	char blkg_path[128];
 
 	/* reference counter (see comments in bfq_bic_update_cgroup) */
-	int ref;
+	refcount_t ref;
 	/* Is bfq_group still online? */
 	bool online;