Re: [PATCH 2/4] arch: Add lightweight memory barriers fast_rmb() and fast_wmb()

Benjamin Herrenschmidt
2014-11-18T21:07:53Z

On Mon, 2014-11-17 at 19:13 -0800, Alexander Duyck wrote:
>
> ARM adds some funky things. They have two different types of
> primitives, a dmb() which is a data memory barrier, and a dsb() which is
> a data synchronization barrier. Then with each of those they have the
> "domains" the barriers are effective within.
>
> So for example on ARM a rmb() is dsb(sy) which means it is a system wide
> synchronization barrier which stops execution on the CPU core until the
> read completes.

That's amazingly heavy handed ... I can see that being useful for MMIO,
we do something similar in our MMIO accessors by using a special variant
of trap instruction that never traps to make the core thing the load
value has been consumed. But that's typically only needed to guarantee
MMIO timings.

> However the smp_rmb() is a dmb(ish) which means it is
> only a barrier as far as the inner shareable domain which I believe only
> goes as far as the local shared cache hierarchy and only guarantees read
> ordering without necessarily halting the CPU or stopping in-order
> speculative reads. So what a coherent_rmb() would be in my setup is
> dmb(sy) which means the barrier runs all the way out to memory, and it
> is allowed to speculative read as long as it does it in order.

Correct, which is thus the same as smp_rmb() ... which was my original
point, or am I missing something else ?

> If it is still unclear you might check out Will Deacon's talk on the
> topic at https://www.youtube.com/watch?v=6ORn6_35kKo, at about 7:00 in
> he explains the whole domains thing, and at 13:30 he explains dmb()/dsb().

Ok, I'll try to watch that when I get a chance.

Cheers,
Ben.