On Thu, 30 Mar 2006, Chen, Kenneth W wrote:
> > > Option (1):
> > >
> > > #define clear_bit clear_bit_mode(..., RELEASE)
> > > #define Smp_mb__before_clear_bit do { } while (0)
> > > #define Smp_mb__after_clear_bit smp_mb()
> > >
> > > Or option (2):
> > >
> > > #define clear_bit clear_bit_mode(..., ACQUIRE)
> > > #define Smp_mb__before_clear_bit smp_mb()
> > > #define Smp_mb__after_clear_bit do { } while (0)
> > >
> > > I'm fine with either one.
> >
> > Neither one is correct because there will always be one combination of
> > clear_bit with these macros that does not generate the required memory
> > barrier.
>
> Can you give an example? Which combination?
For Option(1)
smp_mb__before_clear_bit()
clear_bit(...)(
For Option(2)
clear_bit()
smb_mp_after_clear_bit();
Both have either acquire or release semantics but do not have the effect
of a barrier as required by the macros.
Note that both before and after are used in the core kernel code. Both
must work correctly.
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