[PATCH] Document Linux's memory barriers [try #5]

[Paul E. McKenney]

On Thu, Mar 23, 2006 at 06:34:27PM +0000, David Howells wrote:
> Paul E. McKenney <paulmck at us.ibm.com> wrote:
> 
> > smp_mb__before_atomic_dec() and friends as well?
> 
> These seem to be something Sparc64 related; or, at least, Sparc64 seems to do
> something weird with them.
> 
> What are these meant to achieve anyway? They seems to just be barrier() on a
> lot of systems, even SMP ones.

On architectures such as x86 where atomic_dec() implies an smp_mb(),
they do nothing.  On other architectures, they supply whatever memory
barrier is required.

So, on x86:

	smp_mb();
	atomic_dec(&my_atomic_counter);

would result in -two- atomic instructions, but the smp_mb() would be
absolutely required on CPUs with weaker memory-consistency models.
So your choice is to (1) be inefficient on x86 or (2) be unsafe on
weak-memory-consistency systems.  What we can do instead is:

	smp_mb__before_atomic_dec();
	atomic_dec(&my_atomic_counter);

This allows x86 to generate efficient code -and- allows weak-memory
machines (e.g., Alpha, MIPS, PA-RISC(!), ppc, s390, SPARC64) to generate
safe code.

						Thanx, Paul