kern/131342: [nfs] mounting/unmounting of disks causes NFS to fail

[Martin Birgmeier]

I do not think what you write has to do with the error condition I 
describe, for two reasons: first, mountd is only consulted while doing 
mounts, and the client is not doing a mount, but rather only a file 
access on an already mounted file system; and second, this happens when 
I mount a file system on the server which is not exported and thus does 
not cause the exports list to be changed.

Regards,

Martin

On 07/15/11 12:13, Andrey Simonenko wrote:
> On Fri, Jul 08, 2011 at 01:10:03PM +0000, Martin Birgmeier wrote:
>> The following reply was made to PR kern/131342; it has been noted by GNATS.
>>
>> From: Martin Birgmeier<Martin.Birgmeier at aon.at>
>> To: bug-followup at FreeBSD.org
>> Cc:
>> Subject: Re: kern/131342: [nfs] mounting/unmounting of disks causes NFS to
>>   fail
>> Date: Fri, 08 Jul 2011 15:00:03 +0200
>>
>>   This is a friendly reminder that some kind soul with knowledge of the
>>   relevant kernel parts look into this... the error can easily be
>>   reproduced. I just had it on a 7.4 system which did heavy reading from
>>   an 8.2 server. When I mounted something on the server, the client got a
>>   "Permission denied" reply.
>>
>>   So, to recap the scenario:
>>
>>   7.4 NFS client
>>   8.2 NFS server
>>   client mounts a fs from the server (via IPv4, might be interesting to
>>   look at http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/151681, too, but
>>   that is unrelated)
>>   client does heavy i/o on the mounted fs
>>   server does a mount (on its side, in this case it was from an md device)
>>
>>   -->  error: client gets back some NFS error (in this case "permission
>>   denied")
>>
> This is a well-known behaviour of NFS server on FreeBSD when some file
> system is mounted on a server part.  This happens because when mount(8)
> mounts something it sends the SIGHUP signal to mountd(8), it then re-reads
> exports(5) file and loads new settings into NFS server.  Since mountd(8)
> flushes current settings, loads new settings in a step-by-step style, all
> these changes are happened in nonatomic way.  As a result exports settings
> in the NFS server are incomplete in any point of time during configuration
> loading or reloading.
>
> This time window allows "denied" clients to access not allowed parts of
> exported file systems and "allowed" clients can get access denied errors.
>
> If you are interested in this topic, you can look on the kern/136865
> and my messages related to this topic (information in [3] is outdated
> and in [4] is very outdated):
>
> 1. NFS exports atomic and on-the-fly atomic updates
>     http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/136865
>
> 2. NFSE: new NFS exports support for FreeBSD
>     http://nfse.sourceforge.net/
>
> 3. NFS exports atomic and on-the-fly atomic updates
>     http://lists.freebsd.org/pipermail/freebsd-hackers/2009-June/028782.html
>
> 4. Atomic updates of NFS export lists
>     http://lists.freebsd.org/pipermail/freebsd-hackers/2006-April/016248.html
>
> 5. CFT: nfse compatible mode with mountd (NFS exports file)
>     http://lists.freebsd.org/pipermail/freebsd-fs/2010-May/008421.html
>
>