I got a reply from the aufs developer very quickly:

====================================================================
According to your config.gz, the recent unionfs patch is applied to your
kernel which adds a lock to fsstack_copy_inode_size() in
linux/fs/stack.c.
Aufs already acquire the same lock (since 2006) before calling the
function, so the deadlock happens.

You need to modify au_fsstack_copy_inode_size() in
AufsCVS/fs/aufs25/cpup.c, and make '#else' block available.

----------------------------------------------------------------------
static void au_fsstack_copy_inode_size(struct inode *inode,
struct inode *h_inode)
{
/* this spin_lock conflicts the new unionfs patch in -mm tree */
#if LINUX_VERSION_CODE < KERNEL_VERSION(2, 6, 26) || !defined(MmTree)
spin_lock(&inode->i_lock);
fsstack_copy_inode_size(inode, h_inode);
spin_unlock(&inode->i_lock);
#else
fsstack_copy_inode_size(inode, h_inode);
#endif
}
----------------------------------------------------------------------


Junjiro Okajima

====================================================================



I have tested this with the latest aufs and it seems to work. Here is the PKGBUILD I used
(note that the handling of the splice patch has changed too):

pkgname=aufs
pkgver=20080527
pkgrel=1
_kernver='2.6.25-ARCH'
pkgdesc="Another Unionfs Implementation that supports NFS branches"
arch=('i686' 'x86_64')
url="http://aufs.sourceforge.net/"
license=('GPL2')
depends=('kernel26>=2.6.25.4-1' 'kernel26<=2.6.26-0' 'glibc' 'aufs-utils')
install=$pkgname.install
options=(!libtool !makeflags)

_cvsr=":pserver:anonymous:@aufs.cvs.sourceforge.net:/cvsroot/aufs"
_cvsmod="aufs"

build() {
msg "Connecting to $_cvsmod.sourceforge.net CVS server...."
cvs -d$_cvsr checkout -D $pkgver $_cvsmod

msg "CVS checkout done or server timeout"
msg "Starting make..."

rm -rf $startdir/src/$_cvsmod-build
cp -r $startdir/src/$_cvsmod $startdir/src/$_cvsmod-build

cd $startdir/src/$_cvsmod-build

# use splice functions exported by unionfs kernel patch
# - important for loopback fs mounts
sed s'|#if.*KERNEL_VERSION(2, 6, 26).*|#if 0|' -i fs/aufs25/vfsub.h
sed -i 's|CONFIG_AUFS_SPLICE_PATCH =|CONFIG_AUFS_SPLICE_PATCH = y|' \
local.mk || return 1

# patch for spin_lock conflict with the new unionfs patch
sed s'|#if.*KERNEL_VERSION(2, 6, 26).*|#if 0|' -i fs/aufs25/cpup.c

sed -i 's|.*CONFIG_AUFS_SHWH =.*|CONFIG_AUFS_SHWH = y|' \
local.mk || return 1

# configure for NFS by:
# 1) configure local.mk to use FILP and LHASH
sed -i 's|CONFIG_AUFS_PUT_FILP_PATCH =|CONFIG_AUFS_PUT_FILP_PATCH = y|' \
local.mk || return 1
sed -i 's|CONFIG_AUFS_LHASH_PATCH =|CONFIG_AUFS_LHASH_PATCH = y|' \
local.mk || return 1
# 2) configure local.mk NOT to use FAKE_DM
sed -i 's|CONFIG_AUFS_FAKE_DM = y|CONFIG_AUFS_FAKE_DM = |' \
local.mk || return 1

# build
make KDIR=/usr/src/linux-$_kernver/ -f local.mk || return 1

# install
install -D -m644 fs/aufs25/aufs.ko \
$startdir/pkg/lib/modules/$_kernver/fs/aufs25/aufs.ko || return 1

# tweak the install script for the right kernel version
sed -i -e "s/KERNEL_VERSION=.*/KERNEL_VERSION=${_kernver}/g" \
$startdir/$pkgname.install || return 1
}

Adding Paul to see if we can fix this at the AUFS level, instead of the kernel level.

The above has been confirmed to fix the problem. I'll try to get new packages out tonight, for this and maybe a few other things!

updated 20080527 aufs{,-utils} are now in testing
should be moved out today to extra

Thanks for all the input on this one. Please let me know if this fixes your problems. We'll be testing separately today.