linux-kernel - Re: [RFC] Potential problem in qspinlock due to mixed-size accesses

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Message-ID: <c97665c6-2d8b-49ae-acc5-be5be04f0093@tu-bs.de>
Date: Thu, 19 Jun 2025 16:59:38 +0200
From: Thomas Haas <t.haas@...bs.de>
To: Alan Stern <stern@...land.harvard.edu>
CC: Andrea Parri <parri.andrea@...il.com>, Peter Zijlstra
	<peterz@...radead.org>, Will Deacon <will@...nel.org>, Boqun Feng
	<boqun.feng@...il.com>, Nicholas Piggin <npiggin@...il.com>, David Howells
	<dhowells@...hat.com>, Jade Alglave <j.alglave@....ac.uk>, Luc Maranget
	<luc.maranget@...ia.fr>, "Paul E. McKenney" <paulmck@...nel.org>, Akira
 Yokosawa <akiyks@...il.com>, Daniel Lustig <dlustig@...dia.com>, Joel
 Fernandes <joelagnelf@...dia.com>, <linux-kernel@...r.kernel.org>,
	<linux-arch@...r.kernel.org>, <lkmm@...ts.linux.dev>,
	<hernan.poncedeleon@...weicloud.com>, <jonas.oberhauser@...weicloud.com>,
	"r.maseli@...bs.de" <r.maseli@...bs.de>
Subject: Re: [RFC] Potential problem in qspinlock due to mixed-size accesses

On 19.06.25 16:32, Alan Stern wrote:
> On Thu, Jun 19, 2025 at 04:27:56PM +0200, Thomas Haas wrote:
>> I support this endeavor, but from the Dartagnan side :).
>> We already support MSA in real C/Linux code and so extending our supported
>> Litmus fragment to MSA does not sound too hard to me.
>> We are just missing a LKMM cat model that supports MSA.
> 
> To me, it doesn't feel all that easy.  I'm not even sure where to start
> changing the LKMM.\
> 
> Probably the best way to keep things organized would be to begin with
> changes to the informal operational model and then figure out how to
> formalize them.  But what changes are needed to the operational model?
> 
> Alan

Of course, the difficult part is to get the model right. Maybe I 
shouldn't have said that we are "just" missing the model :).
I'm only saying that we already have some tooling to validate changes to 
the formal model.

I think it makes sense to first play around with the tooling and changes 
to the formal model to just get a feeling of what can go wrong and what 
needs to go right. Then it might become more clear on how the informal 
operational model needs to change.

A good starting point might be to lift the existing ARM8 MSA litmus 
tests to corresponding C/LKMM litmus tests and go from there.
If the informal operational model fails to explain them, then it needs 
to change. This goes only one way though: if ARM permits a behavior then 
so should LKMM. If ARM does not, then it is not so clear if LKMM should 
or not.

Thomas