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

The volatile cast should not disable all that many optimizations,
for example, it is much less hurtful than barrier().  Furthermore,
the main optimizations disabled (pulling atomic_read() and atomic_set()
out of loops) really do need to be disabled.

In many cases you do not need a barrier. Having volatile there *will* 
impact optimization because the compiler cannot use a register that may 
contain the value that was fetched earlier. And the compiler cannot choose 
freely when to fetch the value. The order of memory accesses are fixed if 
you use volatile. If the variable is not volatile then the compiler can 
arrange memory accesses any way they fit and thus generate better code.