From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.deacon@arm.com (Will Deacon)
Date: Mon, 26 Feb 2018 10:39:16 +0000
Subject: [RFC PATCH] riscv/locking: Strengthen spin_lock() and
 spin_unlock()
In-Reply-To: <563431d0-4fb5-9efd-c393-83cc5197e934@nvidia.com>
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 <20180222141249.GA14033@andrea>
 <82beae6a-2589-6136-b563-3946d7c4fc60@nvidia.com>
 <20180222181317.GI2855@linux.vnet.ibm.com>
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 <563431d0-4fb5-9efd-c393-83cc5197e934@nvidia.com>
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To: linux-riscv@lists.infradead.org
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On Thu, Feb 22, 2018 at 11:47:57AM -0800, Daniel Lustig wrote:
> On 2/22/2018 10:27 AM, Peter Zijlstra wrote:
> > On Thu, Feb 22, 2018 at 10:13:17AM -0800, Paul E. McKenney wrote:
> >> So we have something that is not all that rare in the Linux kernel
> >> community, namely two conflicting more-or-less concurrent changes.
> >> This clearly needs to be resolved, either by us not strengthening the
> >> Linux-kernel memory model in the way we were planning to or by you
> >> strengthening RISC-V to be no weaker than PowerPC for these sorts of
> >> externally viewed release-acquire situations.
> >>
> >> Other thoughts?
> > 
> > Like said in the other email, I would _much_ prefer to not go weaker
> > than PPC, I find that PPC is already painfully weak at times.
> 
> Sure, and RISC-V could make this work too by using RCsc instructions
> and/or by using lightweight fences instead.  It just wasn't clear at
> first whether smp_load_acquire() and smp_store_release() were RCpc,
> RCsc, or something else, and hence whether RISC-V would actually need
> to use something stronger than pure RCpc there.  Likewise for
> spin_unlock()/spin_lock() and everywhere else this comes up.
> 
> As Paul's email in the other thread observed, RCpc seems to be
> OK for smp_load_acquire()/smp_store_release() at least according
> to the current LKMM herd spec.  Unlock/lock are stronger already
> I guess.  But if there's an active proposal to strengthen them all
> to something stricter than pure RCpc, then that's good to know.
> 
> My understanding from earlier discussions is that ARM has no plans
> to use their own RCpc instruction for smp_load_acquire() instead
> of their RCsc instructions.  Is that still true?  If they were to
> use the RCpc load there, that would cause them to have the same
> problem we're discussing here, right?  Just checking.

Agreed. No plans to use the LDAPR instruction in Linux.

Will