Re: [v3,11/41] mips: reuse asm-generic/barrier.h

[Will Deacon]

On Tue, Jan 12, 2016 at 12:45:14PM -0800, Leonid Yegoshin wrote:
> >The issue I have with the SYNC description in the text above is that it
> >describes the single CPU (program order) and the dual-CPU (confusingly
> >named global order) cases, but then doesn't generalise any further. That
> >means we can't sensibly reason about transitivity properties when a third
> >agent is involved. For example, the WRC+sync+addr test:
> >
> >
> >P0:
> >Wx = 1
> >
> >P1:
> >Rx == 1
> >SYNC
> >Wy = 1
> >
> >P2:
> >Ry == 1
> ><address dep>
> >Rx = 0
> >
> >
> >I can't find anything to forbid that, given the text. The main problem
> >is having the SYNC on P1 affect the write by P0.
> 
> As I understand that test, the visibility of P0: W[x] = 1 is identical to P1
> and P2 here. If P1 got X before SYNC and write to Y after SYNC then
> instruction source register dependency tracking in P2 prevents a speculative
> load of X before P2 obtains Y from the same place as P0/P1 and calculate
> address of X. If some load of X in P2 happens before address dependency
> calculation it's result is discarded.

I don't think the address dependency is enough on its own. By that
reasoning, the following variant (WRC+addr+addr) would work too:


P0:
Wx = 1

P1:
Rx == 1
<address dep>
Wy = 1

P2:
Ry == 1
<address dep>
Rx = 0


So are you saying that this is also forbidden?
Imagine that P0 and P1 are two threads that share a store buffer. What
then?

> Yes, you can't find that in MIPS SYNC instruction description, it is more
> likely in CM (Coherence Manager) area. I just pointed our arch team member
> responsible for documents and he will think how to explain that.

I tried grepping the linked documents for "coherence manager" but couldn't
find anything. Is the description you refer to available anywhere?

Will