From: "Paul E. McKenney" <redacted>
Date: Wed, 11 Apr 2012 17:45:07 -0700

If I am confused about the simple function call, and if control is really
passing via an interrupt or exception, then rcu_irq_enter() should be
called on entry to the interrupt or exception and rcu_irq_exit() should
be called on exit.

Hmm, it seems the convention changed such that platforms aren't
supposed to invoke do_softirq() from their trap return trap any more.
It's handled completely by irq_exit().

When did that start happening? :-)

Anyways I bet that's the problem, sparc64 invokes do_softirq() in it's
trap return path if softirqs are pending, and that doesn't do any
of the RCU frobbing you mention.