Subject: kern/5844: NFS server sends "permission denied" while mound re-read exports
To: None <firstname.lastname@example.org>
From: Manuel Bouyer <email@example.com>
Date: 07/25/1998 14:57:20
>Synopsis: NFS server sends "permission denied" while mound re-read exports
>Responsible: kern-bug-people (Kernel Bug People)
>Arrival-Date: Sat Jul 25 06:05:01 1998
LIP6, Universite Paris VI.
>Release: NetBSD 1.3.2
System: NetBSD antifer.ipv6.lip6.fr 1.3.2 NetBSD 1.3.2 (ANTIFER) #0: Mon Jun 15 14:17:35 MEST 1998 firstname.lastname@example.org:/emul/linux/share/NetBSD/src/sys/arch/i386/compile/ANTIFER i386
There is an annoying race condition in the server code.
While mountd re-read the /etc/exports file and update the
exports in the kernel, the NFS server sends "permission denied"
errors to clients trying to write to the server at this time.
This is a real problem on home-directory NFS servers which also
has a CD-ROM drive: each time you mount a CD-ROM, some users
get randoms "permission denied" :(. This can halt a big, batch
compile, or a slow ftp transfers.
The following reliably triggers the problem. Have a client which
mounts a partition from a NetBSD NFS server. Start copying a bunch
of files with 'cpio -p'. On the server, send a SIGHUP to mountd.
On the client, the cpio dies with
"cpio: write error: Permission denied"
I didn't look at a real fix yet, I guess the NFS maintainer can fix
this quickly. I think we just need to make the kernel either drop
NFS requests, or delay them while mountd is updating the export list
in the kernel. This may require to new messages to the kernel/mountd
interface: "stop NFS server" and "start NFS server", or something