nfs-root: destructive call to __detach_mounts /dev

[Sven Geggus <lists@fuchsschwanzdomain.de>]

Hello,

I have a couple of machines running Debian GNU/Linux 8 using an NFS-ro-mounted
root filesystem.

The systems seem to get bitten by VFS changes in some (unfortunaly
somewhat difficult to reproduce) circumstances.

The effect is, that /dev (dectmpfs) gets unmounted for some strange reason.

I added the following debug code to fs/namespace.c

printk(KERN_DEBUG "%s: %s\n", __func__, dentry->d_name.name);
dump_stack();

And this is what I get if one of those events happen:

__detach_mounts: dev
CPU: 1 PID: 5551 Comm: modtrack Not tainted 4.1.3-debug-00287-g0fe8050-dirty #1
Hardware name: Dell Inc. Precision WorkStation T3400  /0TP412, BIOS A03 01/31/2008
 ffff880127183c18 ffff880127183bc8 ffffffff815605f8 ffff88012bc4fcb0
 ffff880126c16198 ffff880127183bf8 ffffffff81106cf7 000000000000000f
 ffff880126c16198 ffff880127183c18 ffff880127183d00 ffff880127183c48
Call Trace:
 [<ffffffff815605f8>] dump_stack+0x4c/0x6e
 [<ffffffff81106cf7>] __detach_mounts+0x2b/0x12c
 [<ffffffff810ffbb6>] d_invalidate+0x9a/0xc8
 [<ffffffff810f6b13>] lookup_fast+0x1f5/0x26f
 [<ffffffff810f6dba>] do_last.isra.43+0xd6/0x9fb
 [<ffffffff810f916e>] path_openat+0x1d1/0x53e
 [<ffffffff810f9f41>] ? user_path_at_empty+0x63/0x93
 [<ffffffff810f9fe6>] do_filp_open+0x35/0x85
 [<ffffffff811f1893>] ? find_next_zero_bit+0x17/0x1d
 [<ffffffff81104225>] ? __alloc_fd+0xdd/0xef
 [<ffffffff810ec966>] do_sys_open+0x146/0x1d5
 [<ffffffff810eca1f>] SyS_openat+0xf/0x11
 [<ffffffff81565a17>] system_call_fastpath+0x12/0x6a

Regards

Sven

P.S.: Kernel is vanilla 4.1.3 with aufs patches, but aufs is not related in
this problem.

-- 
Der "normale Bürger" ist nicht an der TU Dresden und schreibt auch
nicht mit mutt. (Ulli Kuhnle in de.comp.os.unix.discussion)

/me is giggls@ircnet, http://sven.gegg.us/ on the Web