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

From: Alexander Duyck
Date: Mon Nov 17 2014 - 22:20:17 EST

On 11/17/2014 04:39 PM, Benjamin Herrenschmidt wrote:
On Mon, 2014-11-17 at 12:24 -0800, Alexander Duyck wrote:
Yes and no. So for example on ARM I used the dmb() operation, however
have to use the barrier at the system level instead of just the inner
shared domain. However on many other architectures they are just the
same as the smp_* variants.

Basically the resultant code is somewhere between the smp and non-smp
barriers in terms of what they cover.
There I don't quite follow you. You need to explain better especially in
the documentation because otherwise people will get it wrong...

If it's ordering in the coherent domain, I fail to see how a DMA agent
is different than another processor when it comes to barriers, so I fail
to see the difference with smp_*

I understand the MMIO vs. memory issue, we do have the same on powerpc,
but that other aspect eludes me.


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. 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.

If it is still unclear you might check out Will Deacon's talk on the topic at, at about 7:00 in he explains the whole domains thing, and at 13:30 he explains dmb()/dsb().

- Alex

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