On 11/17/2014 12:04 PM, Benjamin Herrenschmidt wrote:

On Mon, 2014-11-17 at 09:18 -0800, Alexander Duyck wrote:

quoted

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