[BUG][BISECT] NFSv4 root failures after "fs/locks: allow a lock request to block other requests."

[Krzysztof Kozlowski]

Hi,

Bisect pointed commit ce3147990450a68b3f549088b30f087742a08b5d
("fs/locks: allow a lock request to block other requests.") to failure
boot of NFSv4 with root on several boards.

Log is here:
https://krzk.eu/#/builders/21/builds/836/steps/12/logs/serial0

With several errors:
kernel BUG at ../fs/locks.c:336!
Unable to handle kernel NULL pointer dereference at virtual address 00000004

Configuration:
1. exynos_defconfig
2. Arch ARM Linux
3. Boards:
a. Odroid family (ARMv7, octa-core (Cortex-A7+A15), Exynos5422 SoC)
b. Toradex Colibri VF50 (ARMv7, UP, Cortex-A5)
4. Systemd: v236, 238
5. All boards boot from TFTP with NFS root (NFSv4)

On Colibri VF50 I got slightly different errors:
[   11.663204] Internal error: Oops - undefined instruction: 0 [#1] ARM
[   12.455273] Unable to handle kernel NULL pointer dereference at
virtual address 00000004
and only with some specific GCC (v6.3) or with other conditions which
I did not bisect yet. Maybe Colibri's failure is unrelated to that
commit.

Best regards,
Krzysztof