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Date:   Fri, 10 Sep 2021 19:17:43 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Boqun Feng <boqun.feng@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Dan Lustig <dlustig@nvidia.com>, Will Deacon <will@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Peter Anvin <hpa@zytor.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Vince Weaver <vincent.weaver@maine.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jiri Olsa <jolsa@redhat.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Stephane Eranian <eranian@google.com>,
        linux-tip-commits@vger.kernel.org, palmer@dabbelt.com,
        paul.walmsley@sifive.com, mpe@ellerman.id.au
Subject: Re: [tip:locking/core] tools/memory-model: Add extra ordering for
 locks and remove it for ordinary release/acquire
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On Fri, Sep 10, 2021 at 12:36:32PM -0400, Alan Stern wrote:
> --- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
> +++ usb-devel/tools/memory-model/Documentation/explanation.txt
> @@ -1813,15 +1813,16 @@ spin_trylock() -- we can call these thin
>  lock-acquires -- have two properties beyond those of ordinary releases
>  and acquires.
>  
> +First, when a lock-acquire reads from or is po-after a lock-release,
> +the LKMM requires that every instruction po-before the lock-release
> +must execute before any instruction po-after the lock-acquire.  This
> +would naturally hold if the release and acquire operations were on
> +different CPUs and accessed the same lock variable, but the LKMM says
> +it also holds when they are on the same CPU, even if they access
> +different lock variables.  For example:

Could be I don't understand this right, but the way I'm reading it, it
seems to imply RCsc. Which I don't think we're actually asking at this
time.