RE: Synchronizing Bit operations V2

[Chen, Kenneth W]

Nick Piggin wrote on Thursday, March 30, 2006 11:35 PM
> > >The memory ordering that above combination should produce is a
> > >Linux style smp_mb before the clear_bit. Not a release.
> > 
> > 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.

The usage so far that I can see for

  smp_mb__before_clear_bit()
  clear_bit

is to close a critical section with clear_bit.  I will be hard impressed
to see a usage that allows stuff follows clear_bit to pass clear_bit, but
not to pass the smp_mb_before_xxx.

<end of critical section>
  smp_mb_before_clear_bit
  clear_bit
<begin other code>

But if you stand on the ground of smp_mb_before_xxx protects clear_bit
from occurring before the "end of critical section", then smp_mb_before
is such a brain dead interface and it is another good reason for having
an explicit ordering mode built into the clear_bit. 
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