Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG

Paul E. McKenney
2017-11-20T20:52:09Z

On Mon, Nov 20, 2017 at 08:28:06PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 20, 2017 at 06:05:55PM +0000, Will Deacon wrote:
> > This is a thorny issue, but RCU (specifically rcu_dereference but probably
> > also some READ_ONCEs) relies on being able to utilise syntactic dependency
> > chains to order local accesses to shared variables.
>
> Well, we used to have READ_ONCE() and smp_read_barrier_depends(), but
> we recently munged them together, in the process getting rid of
> lockless_dereference().
>
> So for sure, READ_ONCE() must be able to do the address dependency
> thing, otherwise tons of code comes apart.

I do hope that they track the volatile accesses produced by READ_ONCE().
Otherwise, it would not be good to apply this to anything touching MMIO.

Thanx, Paul