On Mon, Sep 21, 2015 at 11:23:01PM +0100, Will Deacon wrote:
> On Mon, Sep 21, 2015 at 03:10:38PM +0100, Boqun Feng wrote:
> > On Mon, Sep 21, 2015 at 09:45:15PM +0800, Boqun Feng wrote:
> > > On Thu, Sep 17, 2015 at 07:00:01PM +0100, Will Deacon wrote:
> > > > On Thu, Sep 17, 2015 at 03:50:12AM +0100, Boqun Feng wrote:
> > > > > If an ACQUIRE loads the value of stored by a RELEASE, then after the
> > > > > ACQUIRE operation, the CPU executing the ACQUIRE operation will perceive
> > > > > all the memory operations that have been perceived by the CPU executing
> > > > > the RELEASE operation before the RELEASE operation.
> > > > >
> > > > > Which means a release+acquire pair to the same variable guarantees
> > > > > transitivity.
> > > >
> > > > Almost, but on arm64 at least, "all the memory operations" above doesn't
> > > > include reads by other CPUs. I'm struggling to figure out whether that's
> > > > actually an issue.
> > > >
> > >
> > > Ah.. that's indeed an issue! for example:
> > >
> > > CPU 0 CPU 1 CPU 2
> > > ===================== ========================== ================
> > > {a = 0, b = 0, c = 0}
> > > r1 = READ_ONCE(a); WRITE_ONCE(b, 1); r3 = smp_load_acquire(&c);
> > > smp_rmb(); smp_store_release(&c, 1); WRITE_ONCE(a, 1);
> > > r2 = READ_ONCE(b)
> > >
> > > where r1 == 1 && r2 == 0 && r3 == 1 is actually not prohibitted, at
> > > least on POWER.
> > >
> >
> > Oops.. I use wrong litmus here.. so this is prohibitted on POWER. Sorry
> > for the misleading. How about the behavior of that on arm and arm64?
>
> That explicit test is forbidden on arm/arm64 because of the smp_rmb(),
> but if you rewrite it as (LDAR is acquire, STLR is release):
>
>
> {
> 0:X1=x; 0:X3=y;
> 1:X1=y; 1:X2=z;
> 2:X1=z; 2:X3=x;
> }
> P0 | P1 | P2 ;
> LDAR W0,[X1] | MOV W0,#1 | LDAR W0,[X1] ;
> LDR W2,[X3] | STR W0,[X1] | MOV W2,#1 ;
> | STLR W0,[X2] | STR W2,[X3] ;
>
> Observed
> 0:X0=1; 0:X2=0; 2:X0=1;
>
>
> then it is permitted on arm64. Note that herd currently claims that this
> is forbidden, but I'm talking to the authors about getting that fixed :)
But a pure store-release/load-acquire chain would be forbidden in
hardware as well as by herd, correct?
Thanx, Paul
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