kernel: fix hlist_bl again

__d_rehash is dereferencing an almost-NULL pointer on my ARM926.
CONFIG_SMP=n and CONFIG_DEBUG_SPINLOCK=y.

The faulting instruction is:    strne   r3, [r2, #4]
and as can be seen from the register dump below, r2 is 0x00000001, hence
the faulting 0x00000005 address.

__d_rehash is essentially:

       spin_lock_bucket(b);
       entry->d_flags &= ~DCACHE_UNHASHED;
       hlist_bl_add_head_rcu(&entry->d_hash, &b->head);
       spin_unlock_bucket(b);

which is:

       bit_spin_lock(0, (unsigned long *)&b->head.first);
       entry->d_flags &= ~DCACHE_UNHASHED;
       hlist_bl_add_head_rcu(&entry->d_hash, &b->head);
       __bit_spin_unlock(0, (unsigned long *)&b->head.first);

bit_spin_lock(0, ptr) sets bit 0 of *ptr, in this case b->head.first if
CONFIG_SMP or CONFIG_DEBUG_SPINLOCK is set:

#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
       while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
               while (test_bit(bitnum, addr)) {
                       preempt_enable();
                       cpu_relax();
                       preempt_disable();
               }
       }
#endif

So, b->head.first starts off NULL, and becomes a non-NULL (address 1).
hlist_bl_add_head_rcu() does this:

static inline void hlist_bl_add_head_rcu(struct hlist_bl_node *n,
                                       struct hlist_bl_head *h)
{
       first = hlist_bl_first(h);
       n->next = first;
       if (first)
               first->pprev = &n->next;

It is the store to first->pprev which is faulting.

hlist_bl_first():

static inline struct hlist_bl_node *hlist_bl_first(struct hlist_bl_head *h)
{
       return (struct hlist_bl_node *)
               ((unsigned long)h->first & ~LIST_BL_LOCKMASK);
}

but:
#if defined(CONFIG_SMP)
#define LIST_BL_LOCKMASK        1UL
#else
#define LIST_BL_LOCKMASK        0UL
#endif

So, we have one piece of code which sets bit 0 of addresses, and another
bit of code which doesn't clear it before dereferencing the pointer if
!CONFIG_SMP && CONFIG_DEBUG_SPINLOCK.  With the patch below, I can again
sucessfully boot the kernel on my Versatile PB/926 platform.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

include/linux/list_bl.h

@@ -16,7 +16,7 @@
  * some fast and compact auxiliary data.
  */
 
-#if defined(CONFIG_SMP)
+#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
 #define LIST_BL_LOCKMASK	1UL
 #else
 #define LIST_BL_LOCKMASK	0UL