linux-kernel - Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks

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Date:   Thu, 21 Jun 2018 20:04:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Will Deacon <will.deacon@....com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by
 release-acquire and by locks

On Thu, Jun 21, 2018 at 01:27:12PM -0400, Alan Stern wrote:
> More than one kernel developer has expressed the opinion that the LKMM
> should enforce ordering of writes by release-acquire chains and by
> locking.  In other words, given the following code:
> 
> 	WRITE_ONCE(x, 1);
> 	spin_unlock(&s):
> 	spin_lock(&s);
> 	WRITE_ONCE(y, 1);
> 
> or the following:
> 
> 	smp_store_release(&x, 1);
> 	r1 = smp_load_acquire(&x);	// r1 = 1
> 	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 or be part of
> the release-acquire chain.  In terms of the memory model, this means
> that rel-rf-acq-po should be part of the cumul-fence relation.
> 
> All the architectures supported by the Linux kernel (including RISC-V)
> do behave this way, albeit for varying reasons.  Therefore this patch
> changes the model in accordance with the developers' wishes.
> 
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>

Thanks for that Alan!

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>