We occasionally (about once every 2-4 weeks on 1 of a 100 machenes) get 

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000548
  IP: [<ffffffffa0651744>] nfs_delegation_find_inode+0x64/0x150 [nfsv4]

the attached bug is from 4.1.8-100.fc21, but I have seen it on 4.1.5-100.fc21 as
well. Right now I have a realtime modified (xenomai.org) 3.8.13 system that exhibits
the problem more frequently, and that leads me to belive that the problem is
a data race problem, and by instrumenting fs/nfs/delegation.c (3.8.13) to:


  static struct inode *
  nfs_delegation_find_inode_server(struct nfs_server *server,
				 const struct nfs_fh *fhandle)
  {
	  struct nfs_delegation *delegation;
	  struct inode *res = NULL;

 	  printk(KERN_ERR "server = %p\n", server);
	  list_for_each_entry_rcu(delegation, &server->delegations, super_list) {
		  printk(KERN_ERR "delegation = %p\n", delegation);
		  printk(KERN_ERR "delegation->lock = %p\n", delegation->lock);
		  spin_lock(&delegation->lock);
		  printk(KERN_ERR "delegation->inode = %p\n", delegation->inode);
		  if (delegation->inode != NULL) {
			  printk(KERN_ERR "NFS_I(delegation->inode) = %p", NFS_I(delegation->inode));
			  printk(KERN_ERR "NFS_I(delegation->inode)->fh = %p", NFS_I(delegation->inode)->fh);
		  }
		  if (delegation->inode != NULL &&
		      nfs_compare_fh(fhandle, &NFS_I(delegation->inode)->fh) == 0) {
			  res = igrab(delegation->inode);
		  }
		  spin_unlock(&delegation->lock);
		  if (res != NULL)
			  break;
	  }
	  return res;
  }

the system dies with (delegation.c compiled with -O0):

  server = ffff8803dee58458
  delegation =           (null)
  BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
  IP: [<ffffffffa08924ae>] nfs_delegation_find_inode_server+0x80/0x1e0 [nfsv4]

Anybody thet can give me a hint how to write a program that gives rise to multiple 
delegations to further investigate this issue?

Regards

Anders Blomdell

-- 
Anders Blomdell                  Email: anders.blomdell@control.lth.se
Department of Automatic Control
Lund University                  Phone:    +46 46 222 4625
P.O. Box 118                     Fax:      +46 46 138118
SE-221 00 Lund, Sweden