RE: Synchronizing Bit operations V2

["Chen, Kenneth W"]

Nick Piggin wrote on Thursday, March 30, 2006 11:35 PM
> > Whoever designed the smp_mb_before/after_* clearly understand the
> > difference between a bidirectional smp_mb() and a one-way memory
> > ordering.  If smp_mb_before/after are equivalent to smp_mb, what's
> > the point of introducing another interface?
> > 
> 
> They are not. They provide equivalent barrier when performed
> before/after a clear_bit, there is a big difference.

Just to give another blunt brutal example, what is said here is equivalent
to say kernel requires:

   <end of critical section>
   smp_mb_before_spin_unlock
   spin_unlock

Because it is undesirable to have spin_unlock to leak into the critical
Section and allow critical section to leak after spin_unlock.  This is
just plain brain dead.
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