On Wed, 15 Aug 2007, Paul E. McKenney wrote:

Seems to me that we face greater chance of confusion without the
volatile than with, particularly as compiler optimizations become
more aggressive.  Yes, we could simply disable optimization, but
optimization can be quite helpful.

A volatile default would disable optimizations for atomic_read. 
atomic_read without volatile would allow for full optimization by the 
compiler. Seems that this is what one wants in many cases.