linux-kernel - Re: [syzbot] [nfs?] INFO: task hung in nfsd

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Message-ID: <42600ff8-512f-bea1-848c-2cc1c823cb76@afaics.de>
Date: Sat, 21 Sep 2024 09:58:55 +0200
From: Harald Dunkel <harri@...ics.de>
To: NeilBrown <neilb@...e.de>,
 syzbot <syzbot+b568ba42c85a332a88ee@...kaller.appspotmail.com>
Cc: Jeff Layton <jlayton@...nel.org>, Dai.Ngo@...cle.com,
 chuck.lever@...cle.com, kolga@...app.com, linux-kernel@...r.kernel.org,
 linux-nfs@...r.kernel.org, syzkaller-bugs@...glegroups.com, tom@...pey.com
Subject: Re: [syzbot] [nfs?] INFO: task hung in nfsd_umount

NeilBrown wrote:
> 
> We can guess though.  It isn't waiting for a lock - that would show in
> the above list - so it might be waiting for a wakeup, or might be
> spinning.
> The only wake-up I can imagine is in one of the memory-allocation calls,
> but if the system were running out of memory we would probably see
> messages about that.
> 

I have seen something like this. I am running NFS inside a container,
using legacy cgroup. When it got stuck it claimed I cannot login
into the container due to out of memory. When it happens again, I
can send you the exact error message. The next hung nfsd is overdue,
anyway.

> I wonder if it could be looping in svc_xprt_destroy_all(), and sitting
> in the msleep() when the hang is detected so there are no locks to
> report.   I can't see while it would block there.
> 
> It would really help to get a full task list.
> There is a sysctl for that: /proc/sys/kernel/hung_task_all_cpu_backtrace
> 
> Could that be enabled?
> 

I have enabled it on my NFS server (echo 1 >/proc/.../hung_task_all_cpu_backtrace).


Regards

Harri