linux-kernel - Re: [RFC][PATCH 1/4] tools/memory-model: Legitimize current use of tags in LKMM macros

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Date: Mon, 27 May 2024 11:58:01 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: paulmck@...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, dlustig@...dia.com, joel@...lfernandes.org,
  urezki@...il.com, quic_neeraju@...cinc.com, frederic@...nel.org,
  linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 1/4] tools/memory-model: Legitimize current use of
 tags in LKMM macros

On Mon, May 27, 2024 at 05:22:50PM +0200, Jonas Oberhauser wrote:
> The current macros in linux-kernel.def reference instructions such as
> __xchg{mb} or __cmpxchg{acquire}, which are invalid combinations of tags
> and instructions according to the declarations in linux-kernel.bell.
> This works with current herd7 because herd7 removes these tags anyways
> and does not actually enforce validity of combinations at all.
> 
> If a future herd7 version no longer applies these hardcoded
> transformations, then all currently invalid combinations will actually
> appear on some instruction.
> 
> We therefore adjust the declarations to make the resulting combinations
> valid, by adding the 'mb tag to the set of Accesses and allowing all
> Accesses to appear on all read, write, and RMW instructions.
> 
> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
> ---
>  tools/memory-model/linux-kernel.bell | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/memory-model/linux-kernel.bell b/tools/memory-model/linux-kernel.bell
> index ce068700939c..1b2444cdf8d1 100644
> --- a/tools/memory-model/linux-kernel.bell
> +++ b/tools/memory-model/linux-kernel.bell
> @@ -16,10 +16,11 @@
>  enum Accesses = 'once (*READ_ONCE,WRITE_ONCE*) ||
>  		'release (*smp_store_release*) ||
>  		'acquire (*smp_load_acquire*) ||
> -		'noreturn (* R of non-return RMW *)
> -instructions R[{'once,'acquire,'noreturn}]
> -instructions W[{'once,'release}]
> -instructions RMW[{'once,'acquire,'release}]
> +		'noreturn (* R of non-return RMW *) ||
> +		'mb (*xchg(),compare_exchange(),...*)

It would be better to write cmpxchg() instead of compare_exchange().

Alan

> +instructions R[Accesses]
> +instructions W[Accesses]
> +instructions RMW[Accesses]
>  
>  enum Barriers = 'wmb (*smp_wmb*) ||
>  		'rmb (*smp_rmb*) ||
> -- 
> 2.34.1
>