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Date: Tue, 4 Jun 2024 21:28:42 -0700
From: Boqun Feng <boqun.feng@gmail.com>
To: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc: paulmck@kernel.org, stern@rowland.harvard.edu, parri.andrea@gmail.com,
	will@kernel.org, peterz@infradead.org, npiggin@gmail.com,
	dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
	akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
	urezki@gmail.com, quic_neeraju@quicinc.com, frederic@kernel.org,
	linux-kernel@vger.kernel.org, Viktor Vafeiadis <viktor@mpi-sws.org>
Subject: Re: [PATCHv2 3/4] tools/memory-model: Define effect of Mb tags on
 RMWs in tools/...
Message-ID: <Zl_pehpjHVt2z95p@Boquns-Mac-mini.home>
References: <20240604152922.495908-1-jonas.oberhauser@huaweicloud.com>
 <20240604160440.498332-1-jonas.oberhauser@huaweicloud.com>
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In-Reply-To: <20240604160440.498332-1-jonas.oberhauser@huaweicloud.com>

On Tue, Jun 04, 2024 at 06:04:40PM +0200, Jonas Oberhauser wrote:
> Herd7 transforms successful RMW with Mb tags by inserting smp_mb() fences
> around them. We emulate this by considering imaginary po-edges before the
> RMW read and before the RMW write, and extending the smp_mb() ordering
> rule, which currently only applies to real po edges that would be found
> around a really inserted smp_mb(), also to cases of the only imagined po
> edges.
> 
> Reported-by: Viktor Vafeiadis <viktor@mpi-sws.org>
> Suggested-by: Alan Stern <stern@rowland.harvard.edu>
> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
> ---
>  tools/memory-model/linux-kernel.cat | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> index adf3c4f41229..d7e7bf13c831 100644
> --- a/tools/memory-model/linux-kernel.cat
> +++ b/tools/memory-model/linux-kernel.cat
> @@ -34,6 +34,16 @@ 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]) |
> +	(*
> +	 * full-barrier RMWs (successful cmpxchg(), xchg(), etc.) act as
> +	 * though there were enclosed by smp_mb().
> +	 * The effect of these virtual smp_mb() is formalized by adding
> +	 * Mb tags to the read and write of the operation, and providing
> +	 * the same ordering as though there were additional po edges
> +	 * between the Mb tag and the read resp. write.
> +	 *)
> +	([M] ; po ; [Mb & R]) |
> +	([Mb & W] ; po ; [M]) |

I couldn't help suggestting:

	([M] ; po ; [Mb & domain(rmw)]) |
	([Mb & range(rmw)] ; po ; [M]) |

, it's a bit more clear to me, but maybe the comment above is good
enough?

Regards,
Boqun

>  	([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
>  	([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
>  	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
> -- 
> 2.34.1
>