nfs locks autofs (unlocked by exportfs -r)

All,

I have a problem with a embedded cpu board (under linux 2.4.18_mvl30)
(base-8000-19 @A IP address). This target includes a nfs server in
order to export /usr/usbflash where /user is a autofs mount point.

When I try to mount (by nfs) the usb stick from another PC (pc-labo18
@B IP address) :
[root@pc-labo18 oeribon]# mount -t nfs @A:/user/usbflash /mnt/oe

All is correct :
[root@base-8000-19 /]# df | grep usb
/dev/sda1 8274108 4 8274104 1% /user/usbflash
[root@base-8000-19 /]# cat /var/lib/nfs/rmtab
@B:/user/usbflash:0x00000001

But if I umount this last /mnt/oe :
[root@pc-labo18 oeribon]# umount /mnt/oe

On the nfs server the automounter keeps /user/usbflash mounted :
[root@base-8000-19 /]# df | grep usb
/dev/sda1 8274108 4 8274104 1% /user/usbflash
But the rmtab is correct :
[root@base-8000-19 /]# cat /var/lib/nfs/rmtab
[root@base-8000-19 /]#

The next command unlocks the automounter :
[root@base-8000-19 /]# exportfs -r
yaffs: dev is 7939 name is "1f:03"
yaffs: Attempting MTD mount on 31.3, "1f:03"
[root@base-8000-19 /]# df | grep usb
[root@base-8000-19 /]#

Do you have any idea about a bad configuration of autofs or nfs server
in order to explain that exportfs -r command is needed to umount the
usb stick ?

Thanks for your help

Kind regards,

Olivier Eribon

PS : The 3 configuration files

[root@base-8000-19 root]# cat /etc/exports
/user/usbflash *(rw,no_root_squash,no_all_squash)

[root@base-8000-19 root]# cat /etc/auto.master
# $Id: auto.master.sh,v 1.5 2006/09/15 09:16:37 oeribon Exp $
# Sample auto.master file
# Format of this file:
# mountpoint map options
# For details of the format look at autofs(5).
#/var/autofs/misc /etc/auto.misc
/user /etc/auto.misc --timeout=3

[root@base-8000-19 root]# cat /etc/auto.misc
# $Id: auto.misc.sh,v 1.13 2007/10/08 06:35:46 otu8000 Exp $
# This is an automounter map and it has the following format
# key [ -mount-options-separated-by-comma ] location
# Details may be found in the autofs(5) manpage
usbflash -fstype=vfat,sync,nodev,nosuid,umask=000 :/dev/
sda1