linux-kernel - Re: [PATCH memory-model 2/4] Documentation/litmus-tests: Demonstrate unordered failing cmpxchg

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Date: Mon, 6 May 2024 15:21:54 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
  linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
  kernel-team@...a.com, mingo@...nel.org, parri.andrea@...il.com,
  will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
  npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
  luc.maranget@...ia.fr, akiyks@...il.com,
  Frederic Weisbecker <frederic@...nel.org>,
  Daniel Lustig <dlustig@...dia.com>, Joel Fernandes <joel@...lfernandes.org>,
  Mark Rutland <mark.rutland@....com>, Jonathan Corbet <corbet@....net>,
  linux-doc@...r.kernel.org
Subject: Re: [PATCH memory-model 2/4] Documentation/litmus-tests: Demonstrate
 unordered failing cmpxchg

On Mon, May 06, 2024 at 11:00:42AM -0700, Paul E. McKenney wrote:
> On Mon, May 06, 2024 at 06:30:45PM +0200, Jonas Oberhauser wrote:
> > Am 5/6/2024 um 12:05 PM schrieb Jonas Oberhauser:
> > > Am 5/2/2024 um 1:21 AM schrieb Paul E. McKenney:
> > > > This commit adds four litmus tests showing that a failing cmpxchg()
> > > > operation is unordered unless followed by an smp_mb__after_atomic()
> > > > operation.
> > > 
> > > So far, my understanding was that all RMW operations without suffix
> > > (xchg(), cmpxchg(), ...) will be interpreted as F[Mb];...;F[Mb].

It's more accurate to say that RMW operations without a suffix that 
return a value will be interpreted that way.  So for example, 
atomic_inc() doesn't imply any ordering, because it doesn't return a 
value.

> > > barriers explicitly inside the cat model, instead of relying on implicit
> > > conversions internal to herd.

Don't the annotations in linux-kernel.def and linux-kernel.bell (like 
"noreturn") already make this explicit?  

I guess the part that is still implicit is that herd7 doesn't regard 
failed RMW operations as actual RMWs (they don't have a store part).

Alan