On Thu, Aug 16, 2007 at 02:11:43PM +1000, Paul Mackerras wrote:

The uses of atomic_read where one might want it to allow caching of
the result seem to me to fall into 3 categories:

1. Places that are buggy because of a race arising from the way it's
   used.

2. Places where there is a race but it doesn't matter because we're
   doing some clever trick.

3. Places where there is some locking in place that eliminates any
   potential race.

Agreed.

In case 1, adding volatile won't solve the race, of course, but it's
hard to argue that we shouldn't do something because it will slow down
buggy code.  Case 2 is hopefully pretty rare and accompanied by large
comment blocks, and in those cases caching the result of atomic_read
explicitly in a local variable would probably make the code clearer.
And in case 3 there is no reason to use atomic_t at all; we might as
well just use an int.

Since adding volatile doesn't help any of the 3 cases, and
takes away optimisations from both 2 and 3, I wonder what
is the point of the addition after all?

Cheers,
-- 
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