linux-kernel - Re: LKMM: Making RMW barriers explicit

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Date: Thu, 16 May 2024 10:44:05 +0200
From: Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
 Alan Stern <stern@...land.harvard.edu>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, linux-kernel@...r.kernel.org,
 linux-arch@...r.kernel.org, kernel-team@...a.com, parri.andrea@...il.com,
 boqun.feng@...il.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
 Joel Fernandes <joel@...lfernandes.org>
Subject: Re: LKMM: Making RMW barriers explicit

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.

Hernan