On Tue, Sep 22, 2015 at 07:26:56AM +0800, Boqun Feng wrote:

On Mon, Sep 21, 2015 at 11:24:27PM +0100, Will Deacon wrote:

quoted

Hi Boqun,

On Sun, Sep 20, 2015 at 09:23:03AM +0100, Boqun Feng wrote:

quoted

On Sat, Sep 19, 2015 at 11:33:10PM +0800, Boqun Feng wrote:

quoted

On Fri, Sep 18, 2015 at 05:59:02PM +0100, Will Deacon wrote:

quoted

On Wed, Sep 16, 2015 at 04:49:31PM +0100, Boqun Feng wrote:

quoted

On powerpc, we don't need a general memory barrier to achieve acquire and
release semantics, so __atomic_op_{acquire,release} can be implemented
using "lwsync" and "isync".

I'm assuming isync+ctrl isn't transitive, so we need to get to the bottom

Actually the transitivity is still guaranteed here, I think ;-)

The litmus test I'm thinking of is:


{
0:r2=x;
1:r2=x; 1:r5=z;
2:r2=z; 2:r4=x;
}
 P0           | P1            | P2           ;
 li r1,1      | lwz r1,0(r2)  | lwz r1,0(r2) ;
 stw r1,0(r2) | cmpw r1,r1    | cmpw r1,r1   ;
              | beq LC00      | beq  LC01    ;
              | LC00:         | LC01:        ;
              | isync         | isync        ;
              | li r4,1       | lwz r3,0(r4) ;
              | stw r4,0(r5)  |              ;
exists
(1:r1=1 /\ 2:r1=1 /\ 2:r3=0)


Which appears to be allowed. I don't think you need to worry about backwards
branches for the ctrl+isync construction (none of the current example do,
afaict).

Yes.. my care of backwards branches is not quite related to the topic, I
concerned that mostly because my test is using atomic operation, and I
just want to test the exact asm code.

quoted

Anyway, all the problematic cases seem to arise when we start mixing
ACQUIRE/RELEASE accesses with relaxed accesses (i.e. where an access from
one group reads from an access in the other group). It would be simplest
to say that this doesn't provide any transitivity guarantees, and that
an ACQUIRE must always read from a RELEASE if transitivity is required.

Agreed. RELEASE alone doesn't provide transitivity and transitivity is

          ^^^^^^^
This should be ACQUIRE...

guaranteed only if an ACQUIRE read from a RELEASE. That's exactly the
direction which the link (https://lkml.org/lkml/2015/9/15/836) is
heading to. So I think we are fine here to use ctrl+isync here, right?

Regards,
Boqun