Re: [PATCH 16/19] context_tracking: Convert state to atomic_t

[Will Deacon]

On Sat, Mar 12, 2022 at 11:54:09PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 02, 2022 at 04:48:07PM +0100, Frederic Weisbecker wrote:
> > +static __always_inline int __ct_state(void)
> > +{
> > +	return atomic_read(this_cpu_ptr(&context_tracking.state));
> > +}
> 
> One arguably horrible thing to do would be to write it like:
> 
> 	return __this_cpu_read(context_tracking.state.counter);
> 
> IIRC that will actually DTRT since atomic_read() is basically defined to
> be READ_ONCE() and this_cpu_read() implies the same.
> 
> Only PowerPC and s390 implement arch_atomic_read() in asm, but I don't
> think they have a particularly good reason to. The only other weird case
> is Alpha, where READ_ONCE() implies smp_mb() because Alpha. I'm not sure
> we care about that case, hmm?

If we don't care about the dependency ordering, then __READ_ONCE() is the
chappy to use if the types don't get in the way too much.

Will