Created attachment 474201 [details]
patch for linux-2.6.32-71.el6.x86_64

Description of problem:

When mounting NFSv4 filesystem, NFS client's kernel panic for a BUG_ON at
__rpc_create_common().

  The panic place is:
    rpc_mkpipe
      __rpc_lookup_create()          <=== find pipefile *idmap*
      __rpc_mkpipe()                 <=== pipefile is *idmap*
        __rpc_create_common()
          ******  BUG_ON(!d_unhashed(dentry)); ******    *panic*

Version-Release number of selected component (if applicable):

Red Hat Enterprise Linux 6.0
kernel-2.6.32-71

How reproducible:
# cat /etc/exports
/nfsv4        *(rw,insecure,no_root_squash,fsid=0)

# mkdir -p /nfsv4

# service nfs restart

# cat mount-panic.sh
#!/bin/sh
LOOPTOTAL=768
LOOPCOUNT=0
while [ $LOOPCOUNT -ne $LOOPTOTAL ]
do
        ((LOOPCOUNT += 1))
        service nfs restart
        /usr/sbin/rpc.idmapd
        mount -t nfs4 127.0.0.1:/ /mnt || return;
        ls -l /var/lib/nfs/rpc_pipefs/nfs/*
        umount /mnt;
        echo $LOOPCOUNT
done

# sh mount-panic.sh

Actual results:
Panic happens while the above script is running.

Expected results:
The script completes.

Additional info:
It's reported on the linux-nfs community, too.
http://marc.info/?l=linux-nfs&m=129231689022990&w=2

Patch for linux-2.6.32-71.el6.x86_64 is attached.