If a call of the function blkg_lookup_check() failed,

Thanks that you would like to integrate something from my suggestion.

we should be release the previously allocated block group

I find that an other wording would be preferred over “we should”.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?id=b9bbe6ed63b2b9f2c9ee5cbd0f2c946a2723f4ce#n151

before jumping to the lable 'fail_unlock' in the implementation of

Please avoid another typo here.

the function blkg_conf_prep().

Suggested-by: Markus Elfring <redacted>

I find this tag questionable because the bug fix was presented before.
https://lore.kernel.org/linux-block/4c670a6c-98c3-2b14-7438-09199506d92f@web.de/ (local)
https://lkml.org/lkml/2020/5/15/779

---
V2: omit the source code quotation from
    the change description

* I recommend to improve also the patch subject accordingly.

* Would you like to move the name from the previous tag into
  this information?

---
 block/blk-cgroup.c | 1 +

I suggest to omit the triple dashes before this diffstat.

Regards,
Markus