Herbert Xu writes:

Can you find an actual atomic_read code snippet there that is
broken without the volatile modifier?

There are some in arch-specific code, for example line 1073 of
arch/mips/kernel/smtc.c.  On mips, cpu_relax() is just barrier(), so
the empty loop body is ok provided that atomic_read actually does the
load each time around the loop.

Paul.