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

From: Leonid Yegoshin
Date: Fri Jan 15 2016 - 13:55:12 EST

On 01/15/2016 01:57 AM, Will Deacon wrote:

I think you figured this out while I was sleeping, but just to confirm:

1. The MIPS64 ISA doc [1] talks about SYNC in a way that applies only
to memory accesses appearing in *program-order* before the SYNC

2. We need WRC+sync+addr to work, which means that the SYNC in P1 must
also capture the store in P0 as being "before" the barrier. Leonid
reckons it works, but his explanation [2] focussed on the address
dependency in P2 as to why this works. If that is the case (i.e.
address dependency provides global transitivity), then WRC+addr+addr
should also work (even though its not required).

No, it is not correct. There is one old design which provides access to core (thread0 + thread1) write-buffers for threads load in advance of it is visible to other cores. It means, that WRC+sync+addr passes because of SYNC in write thread and register dependency inside other thread but WRC+addr+addr may fail because other core may get a stale data.

3. It seems that WRC+addr+addr doesn't work, so I'm still suspicious
about WRC+sync+addr, because neither the architecture document or
Leonid's explanation tell me that it should be forbidden.


[2] (scroll to the end)