On Mon, Sep 21, 2015 at 11:24:27PM +0100, Will Deacon wrote:
> Hi Boqun,
>
> On Sun, Sep 20, 2015 at 09:23:03AM +0100, Boqun Feng wrote:
> > On Sat, Sep 19, 2015 at 11:33:10PM +0800, Boqun Feng wrote:
> > > On Fri, Sep 18, 2015 at 05:59:02PM +0100, Will Deacon wrote:
> > > > On Wed, Sep 16, 2015 at 04:49:31PM +0100, Boqun Feng wrote:
> > > > > 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. > 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 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
signature.asc
Description: PGP signature
_______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
