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

[Jeff Layton]

On Wed, 2018-08-15 at 14:28 +0200, Krzysztof Kozlowski wrote:
> 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

The BUG is due to a lock being freed when the fl_blocked list wasn't
empty (implying that there were still blocked locks waiting on it).

There are a number of calls to locks_delete_lock_ctx in posix_lock_inode
and I don't think the fl_blocked list is being handled properly with all
of them. It only transplants the blocked locks to a new lock when there
are surviving locks on the list, and that may not be the case when the
whole file is being unlocked.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>