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SubjectRE: Fix unlock_buffer() to work the same way as bit_unlock()
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Christoph Lameter wrote on Tuesday, March 28, 2006 3:48 PM
> On Tue, 28 Mar 2006, Zoltan Menyhart wrote:
>
> > Why not to use separate bit operations for different purposes?
> >
> > - e.g. "test_and_set_bit_N_acquire()" for lock acquisition
> > - "test_and_set_bit()", "clear_bit()" as they are today
> > - "release_N_clear_bit()"...
> >
>
> That would force IA64 specifics onto all other architectures.
>
> Could we simply define these smb_mb__*_clear_bit to be noops
> and then make the atomic bit ops to have full barriers? That would satisfy
> Nick's objections.
>
> --- linux-2.6.16.orig/include/asm-ia64/bitops.h 2006-03-19 21:53:29.000000000 -0800
> +++ linux-2.6.16/include/asm-ia64/bitops.h 2006-03-28 15:45:08.000000000 -0800
> @@ -45,6 +45,7 @@
> old = *m;
> new = old | bit;
> } while (cmpxchg_acq(m, old, new) != old);
> + smb_mb();
> }

There are better way to do it. The pointer is already cast as volatile,
so old = *m has acq semantics built-in, we can just change cmpxchg_acq to
cmpxchg_rel, then effectively it is a full memory barrier without doing the
expensive smp_mb().

- Ken
-
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