linux-kernel - Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

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Message-ID: <20210910171743.GO4323@worktop.programming.kicks-ass.net>
Date:   Fri, 10 Sep 2021 19:17:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Dan Lustig <dlustig@...dia.com>, Will Deacon <will@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Anvin <hpa@...or.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Olsa <jolsa@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        linux-tip-commits@...r.kernel.org, palmer@...belt.com,
        paul.walmsley@...ive.com, mpe@...erman.id.au
Subject: Re: [tip:locking/core] tools/memory-model: Add extra ordering for
 locks and remove it for ordinary release/acquire

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.