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Date: Thu, 23 May 2024 12:05:52 -0400
From: Alan Stern <stern@rowland.harvard.edu>
To: Hernan Ponce de Leon <hernan.poncedeleon@huaweicloud.com>
Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
  "Paul E. McKenney" <paulmck@kernel.org>, linux-kernel@vger.kernel.org,
  linux-arch@vger.kernel.org, kernel-team@meta.com, parri.andrea@gmail.com,
  boqun.feng@gmail.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
  Joel Fernandes <joel@joelfernandes.org>
Subject: Re: LKMM: Making RMW barriers explicit
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On Thu, May 23, 2024 at 04:26:23PM +0200, Hernan Ponce de Leon wrote:
> On 5/23/2024 4:05 PM, Alan Stern wrote:
> > Overall, it seems better to have herd7 assign the right tag, but change
> > the way the .def file works so that it can tell herd7 which tag to use
> > in each of the success and failure cases.
> 
> I am not fully sure how herd7 uses the .def file, but I guess something like
> adding a second memory tag to __cmpxchg could work
> 
> cmpxchg(X,V,W) __cmpxchg{mb, once}(X,V,W)
> cmpxchg_relaxed(X,V,W) __cmpxchg{once, once}(X,V,W)
> cmpxchg_acquire(X,V,W) __cmpxchg{acquire, acquire}(X,V,W)
> cmpxchg_release(X,V,W) __cmpxchg{release, release}(X,V,W)

Right, except that the last two should be:

cmpxchg_acquire(X,V,W) __cmpxchg{acquire, once}(X,V,W)
cmpxchg_release(X,V,W) __cmpxchg{release, once}(X,V,W)

Alan