Re: LKMM: Making RMW barriers explicit

Hernan Ponce de Leon
2024-05-21T11:39:02Z

On 5/18/2024 2:31 AM, Alan Stern wrote:
> On Thu, May 16, 2024 at 10:44:05AM +0200, Hernan Ponce de Leon wrote:
>> On 5/16/2024 10:31 AM, Jonas Oberhauser wrote:
>>>
>>>
>>> Am 5/16/2024 um 3:43 AM schrieb Alan Stern:
>>>> Hernan and Jonas:
>>>>
>>>> Can you explain more fully the changes you want to make to herd7 and/or
>>>> the LKMM?  The goal is to make the memory barriers currently implicit in
>>>> RMW operations explicit, but I couldn't understand how you propose to do
>>>> this.
>>>>
>>>> Are you going to change herd7 somehow, and if so, how?  It seems like
>>>> you should want to provide sufficient information so that the .bell
>>>> and .cat files can implement the appropriate memory barriers associated
>>>> with each RMW operation.  What additional information is needed?  And
>>>> how (explained in English, not by quoting source code) will the .bell
>>>> and .cat files make use of this information?
>>>>
>>>> Alan
>>>
>>>
>>> I don't know whether herd7 needs to be changed. Probably, herd7 does the
>>> following:
>>> - if a tag called Mb appears on an rmw instruction (by instruction I
>>> mean things like xchg(), atomic_inc_return_relaxed()), replace it with
>>> one of those things:
>>>   * full mb ; once (the rmw) ; full mb, if a value returning
>>> (successful) rmw
>>>   * once (the rmw)   otherwise
>>> - everything else gets translated 1:1 into some internal representation
>>
>> This is my understanding from reading the source code of CSem.ml in herd7's
>> repo.
>>
>> Also, this is exactly what dartagnan is currently doing.
>>
>>>
>>> What I'm proposing is:
>>> 1. remove this transpilation step,
>>> 2. and instead allow the Mb tag to actually appear on RMW instructions
>>> 3. change the cat file to explicitly define the behavior of the Mb tag
>>> on RMW instructions
>>
>> These are the exact 3 things I changed in dartagnan for testing what Jonas
>> proposed.
>>
>> I am not sure if further changes are needed for herd7.

I implemented these changes in herd7 and they seem enough.
I opened a PRs for discussion
https://github.com/herd/herdtools7/pull/865

>
> Okay, good. This answers the first part of what I asked. What about
> the second part? That is, how will the changes to the .def, .bell, and
> .cat files achieve your goals?

At least the cat model needs to be updated. If I remove the fences from
herd7, but keep the current model, these 4 tests fail.

--- Summary:
!!! Result changed:
./litmus/manual/locked/SUW+or-ow+l-ow-or.litmus.out.new
!!! Result changed:
./litmus/manual/atomic/C-PaulEMcKenney-SB+adat-o+adat-o.litmus.out.new
!!! Result changed: ./litmus/manual/atomic/C-atomic-01.litmus.out.new
!!! Result changed: ./litmus/manual/atomic/C-atomic-02.litmus.out.new

Using this patch I get the correct results

diff --git a/tools/memory-model/linux-kernel.cat
b/tools/memory-model/linux-kernel.cat
index adf3c4f41229..7e4787cdbd66 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -34,6 +34,12 @@ let R4rmb = R \ Noreturn (* Reads for which rmb
works *)
let rmb = [R4rmb] ; fencerel(Rmb) ; [R4rmb]
let wmb = [W] ; fencerel(Wmb) ; [W]
let mb = ([M] ; fencerel(Mb) ; [M]) |
+ (* everything across a full barrier RMW is ordered. This
includes up to one event inside the RMW. *)
+ ([M] ; po ; [RMW & Mb] ; po ; [M]) |
+ (* full barrier RMW writes are ordered with everything behind
the RMW *)
+ ([W & RMW & Mb] ; po ; [M]) |
+ (* full barrier RMW reads are ordered with everything before the
RMW *)
+ ([M] ; po ; [R & RMW & Mb]) |
([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |

Hernan

>
> Alan