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

[Alexander Duyck <alexander.duyck@gmail.com>]

On 11/17/2014 12:04 PM, Benjamin Herrenschmidt wrote:
> On Mon, 2014-11-17 at 09:18 -0800, Alexander Duyck wrote:
>> There are a number of situations where the mandatory barriers rmb() and
>> wmb() are used to order memory/memory operations in the device drivers
>> and those barriers are much heavier than they actually need to be.  For
>> example in the case of PowerPC wmb() calls the heavy-weight sync
>> instruction when for memory/memory operations all that is really needed is
>> an lsync or eieio instruction.
> So essentially those are the same as the smp_* variants but not nop'ed
> out on !CONFIG_SMP right ? 
>
> Ben.
>

Yes and no.  So for example on ARM I used the dmb() operation, however I
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.

Thanks,

Alex