Re: [syzbot] [nfs?] INFO: task hung in nfsd_umount

[Harald Dunkel <harri@afaics.de>]

Hi Neil,

On 2024-09-29 00:23:18, NeilBrown wrote:
> 
> Thanks for the logs.  The point to flush_workqueue() being a problem,
> presumably from nfsd4_probe_callback_sync(), though I'm not 100% sure of
> that.  Maybe some deadlock in the callback code.  I'm not very familiar
> with that code and nothing immediately jumps out.
> 
> I had thought that hung_task_all_cpu_backtrace would show a backtrace of
> *all* tasks - I missed the "cpu" in there.
> If if it happens again and if you can
>    echo t > /proc/sysrq-trigger
> to get stack traces of everything, that might help.  Maybe it won't be
> necessary if I or someone else can spot a deadlock with
> flush_workqueue().
> 

I just learned that kernel.hung_task_panic = 1 should have been set, too.
Sorry for that. Currently I have

	kernel.hung_task_panic = 1
	kernel.hung_task_all_cpu_backtrace = 1

Please confirm.

I have set a watchdog to run the sysrq trigger on the NFS server if df
on an NFS client doesn't respond.


Regards
Harri