On Fri, 17 Aug 2007, Nick Piggin wrote:

Stefan Richter wrote:
[...]
Just use spinlocks if you're not absolutely clear about potential
races and memory ordering issues -- they're pretty cheap and simple.

I fully agree with this. As Paul Mackerras mentioned elsewhere,
a lot of authors sprinkle atomic_t in code thinking they're somehow
done with *locking*. This is sad, and I wonder if it's time for a
Documentation/atomic-considered-dodgy.txt kind of document :-)

quoted

Sure, now
that I learned of these properties I can start to audit code and insert
barriers where I believe they are needed, but this simply means that
almost all occurrences of atomic_read will get barriers (unless there
already are implicit but more or less obvious barriers like msleep).

You might find that these places that appear to need barriers are
buggy for other reasons anyway. Can you point to some in-tree code
we can have a look at?

Such code was mentioned elsewhere (query nodemgr_host_thread in cscope)
that managed to escape the requirement for a barrier only because of
some completely un-obvious compilation-unit-scope thing. But I find such
an non-explicit barrier quite bad taste. Stefan, do consider plunking an
explicit call to barrier() there.


Satyam