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SubjectRe: [PATCH] arch: Introduce read_acquire()

On 11/12/2014 02:10 AM, Will Deacon wrote:
> On Tue, Nov 11, 2014 at 07:40:22PM +0000, Linus Torvalds wrote:
>> On Tue, Nov 11, 2014 at 10:57 AM, <> wrote:
>>> On reviewing the documentation and code for smp_load_acquire() it occured
>>> to me that implementing something similar for CPU <-> device interraction
>>> would be worth while. This commit provides just the load/read side of this
>>> in the form of read_acquire().
>> So I don't hate the concept, but. there's a couple of reasons to think
>> this is broken.
>> One is just the name. Why do we have "smp_load_acquire()", but then
>> call the non-smp version "read_acquire()"? That makes very little
>> sense to me. Why did "load" become "read"?
> [...]
>> But we do have a very real difference between "smp_rmb()" (inter-cpu
>> cache coherency read barrier) and "rmb()" (full memory barrier that
>> synchronizes with IO).
>> And your patch is very confused about this. In *some* places you use
>> "rmb()", and in other places you just use "smp_load_acquire()". Have
>> you done extensive verification to check that this is actually ok?
>> Because the performance difference you quote very much seems to be
>> about your x86 testing now akipping the IO-synchronizing "rmb()", and
>> depending on DMA being ordered even without it.
>> And I'm pretty sure that's actually fine on x86. The real
>> IO-synchronizing rmb() (which translates into a lfence) is only needed
>> for when you have uncached accesses (ie mmio) on x86. So I don't think
>> your code is wrong, I just want to verify that everybody understands
>> the issues. I'm not even sure DMA can ever really have weaker memory
>> ordering (I really don't see how you'd be able to do a read barrier
>> without DMA stores being ordered natively), so maybe I worry too much,
>> but the ppc people in particular should look at this, because the ppc
>> memory ordering rules and serialization are some completely odd ad-hoc
>> black magic....
> Right, so now I see what's going on here. This isn't actually anything
> to do with acquire/release (I don't know of any architectures that have
> a read-barrier-acquire instruction), it's all about DMA to main memory.

Actually it is sort of, I just hadn't realized it until I read some of
the explanations of the C11 acquire/release memory order specifics, but
I believe most network drivers are engaged in acquire/release logic
because we are usually using something such as a lockless descriptor
ring to pass data back and forth between the device and the system. The
net win for device drivers is that we can remove some of the
heavy-weight barriers that are having to be used by making use of
lighter barriers or primitives such as lwsync vs sync in PowerPC or ldar
vs dsb(ld) on arm64.

> If a device is DMA'ing data *and* control information (e.g. 'descriptor
> valid') to memory, then it must be maintaining order between those writes
> with respect to memory. In that case, using the usual MMIO barriers can
> be overkill because we really just want to enforce read-ordering on the CPU
> side. In fact, I think you could even do this with a fake address dependency
> on ARM (although I'm not actually suggesting we do that).
> In light of that, it actually sounds like we want a new set of barrier
> macros that apply only to DMA buffer accesses by the CPU -- they wouldn't
> enforce ordering against things like MMIO registers. I wonder whether any
> architectures would implement them differently to the smp_* flavours?

My concern would be the cost of the barriers vs the acquire/release
primitives. In the case of arm64 I am assuming there is a reason for
wanting to use ldar vs dsb instructions. I would imagine the devices
drivers would want to get the same kind of advantages.

>> But anything with non-cache-coherent DMA is obviously very suspect too.
> I think non-cache-coherent DMA should work too (at least, on ARM), but
> only for buffers mapped via dma_alloc_coherent (i.e. a non-cacheable
> mapping).
> Will

For now my plan is to focus on coherent memory only with this.
Specifically it is only really intended for use with dma_alloc_coherent.



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