Fix unlock_buffer() to work the same way as bit_unlock()

[Christoph Lameter]

Currently unlock_buffer() contains a smb_mb__after_clear_bit() which is 
weird because bit_spin_unlock() uses smb_mb__before_clear_bit():

>From include/linux/bit_spinlock.h:

static inline void bit_spin_unlock(int bitnum, unsigned long *addr)
{
        smp_mb__before_clear_bit();
        clear_bit(bitnum, addr);
        preempt_enable();
        __release(bitlock);
}

For most architectures there is no difference because both
smp_mb__after_clear_bit() and smp_mb__before_clear_bit() are both
memory barriers and clear_buffer_locked() is an atomic operation.
However, they differ under IA64.

Note that this potential race has never been seen under IA64. It was 
discovered by inspection by Zoltan Menyhart <Zoltan.Menyhart@xxxxxxx>. 

Regardless if this is a true race or not, I think the unlock sequence 
needs to be the same for bit locks and unlock_buffer(). Maybe 
unlock_buffer and lock_buffer better use bit spinlock operations?

Change unlock_buffer() to work the same way as bit_spin_unlock.

Signed-off-by: Christoph Lameter <clameter@xxxxxxx>

Index: linux-2.6/fs/buffer.c
===================================================================
--- linux-2.6.orig/fs/buffer.c	2006-03-27 14:09:54.000000000 -0800
+++ linux-2.6/fs/buffer.c	2006-03-27 19:40:32.000000000 -0800
@@ -78,8 +78,8 @@ EXPORT_SYMBOL(__lock_buffer);
 
 void fastcall unlock_buffer(struct buffer_head *bh)
 {
+	smp_mb__before_clear_bit();
 	clear_buffer_locked(bh);
-	smp_mb__after_clear_bit();
 	wake_up_bit(&bh->b_state, BH_Lock);
 }
 
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