On Tue, Sep 01, 2015 at 08:00:27PM +0100, Will Deacon wrote:
> On Fri, Aug 28, 2015 at 04:39:21PM +0100, Peter Zijlstra wrote:
> > On Fri, Aug 28, 2015 at 10:16:02PM +0800, Boqun Feng wrote:
> > > Ah.. just read through the thread you mentioned, I might misunderstand
> > > you, probably because I didn't understand RCpc well..
> > >
> > > You are saying that in a RELEASE we -might- switch from smp_lwsync() to
> > > smp_mb() semantically, right? I guess this means we -might- switch from
> > > RCpc to RCsc, right?
> > >
> > > If so, I think I'd better to wait until we have a conclusion for this.
> >
> > Yes, the difference between RCpc and RCsc is in the meaning of RELEASE +
> > ACQUIRE. With RCsc that implies a full memory barrier, with RCpc it does
> > not.
>
> We've discussed this before, but for the sake of completeness, I don't
> think we're fully RCsc either because we don't order the actual RELEASE
> operation again a subsequent ACQUIRE operation:
>
> P0
> smp_store_release(&x, 1);
> foo = smp_load_acquire(&y);
>
> P1
> smp_store_release(&y, 1);
> bar = smp_load_acquire(&x);
>
> We allow foo == bar == 0, which is prohibited by SC.
I certainly hope that no one expects foo == bar == 0 to be prohibited!!!
On the other hand, in this case, foo == bar == 1 will be prohibited:
P0
foo = smp_load_acquire(&y);
smp_store_release(&x, 1);
P1
bar = smp_load_acquire(&x);
smp_store_release(&y, 1);
> However, we *do* enforce ordering on any prior or subsequent accesses
> for the code snippet above (the release and acquire combine to give a
> full barrier), which makes these primitives well suited to things like
> message passing.
If I understand your example correctly, neither x86 nor Power implement
a full barrier in this case. For example:
P0
WRITE_ONCE(a, 1);
smp_store_release(b, 1);
r1 = smp_load_acquire(c);
r2 = READ_ONCE(d);
P1
WRITE_ONCE(d, 1);
smp_mb();
r3 = READ_ONCE(a);
Both x86 and Power can reorder P0 as follows:
P0
r1 = smp_load_acquire(c);
r2 = READ_ONCE(d);
WRITE_ONCE(a, 1);
smp_store_release(b, 1);
Which clearly shows that the non-SC outcome r2 == 0 && r3 == 0 is allowed.
Or am I missing your point here?
Thanx, Paul
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