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    SubjectFix unlock_buffer() to work the same way as bit_unlock()
    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@free.fr>.

    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@sgi.com>

    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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