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: <YTiXyiA92dM9726M@hirez.programming.kicks-ass.net>
Date:   Wed, 8 Sep 2021 13:00:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     stern@...land.harvard.edu, alexander.shishkin@...ux.intel.com,
        hpa@...or.com, andrea.parri@...rulasolutions.com, mingo@...nel.org,
        paulmck@...nel.org, vincent.weaver@...ne.edu, tglx@...utronix.de,
        jolsa@...hat.com, acme@...hat.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, eranian@...gle.com, will@...nel.org
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:locking/core] tools/memory-model: Add extra ordering for
 locks and remove it for ordinary release/acquire

On Tue, Oct 02, 2018 at 03:11:10AM -0700, tip-bot for Alan Stern wrote:
> Commit-ID:  6e89e831a90172bc3d34ecbba52af5b9c4a447d1
> Gitweb:     https://git.kernel.org/tip/6e89e831a90172bc3d34ecbba52af5b9c4a447d1
> Author:     Alan Stern <stern@...land.harvard.edu>
> AuthorDate: Wed, 26 Sep 2018 11:29:17 -0700
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Tue, 2 Oct 2018 10:28:01 +0200
> 
> tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
> 
> More than one kernel developer has expressed the opinion that the LKMM
> should enforce ordering of writes by locking.  In other words, given
> the following code:
> 
> 	WRITE_ONCE(x, 1);
> 	spin_unlock(&s):
> 	spin_lock(&s);
> 	WRITE_ONCE(y, 1);
> 
> the stores to x and y should be propagated in order to all other CPUs,
> even though those other CPUs might not access the lock s.  In terms of
> the memory model, this means expanding the cumul-fence relation.

Let me revive this old thread... recently we ran into the case:

	WRITE_ONCE(x, 1);
	spin_unlock(&s):
	spin_lock(&r);
	WRITE_ONCE(y, 1);

which is distinct from the original in that UNLOCK and LOCK are not on
the same variable.

I'm arguing this should still be RCtso by reason of:

  spin_lock() requires an atomic-acquire which:

    TSO-arch)		implies smp_mb()
    ARM64)		is RCsc for any stlr/ldar
    Power)		LWSYNC
    Risc-V)		fence r , rw
    *)			explicitly has smp_mb()


However, Boqun points out that the memory model disagrees, per:

  https://lkml.kernel.org/r/YTI2UjKy+C7LeIf+@boqun-archlinux

Is this an error/oversight of the memory model, or did I miss a subtlety
somewhere?