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Date: Thu, 12 Nov 2015 19:00:03 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: David Daney <ddaney@caviumnetworks.com>
Cc: ralf@linux-mips.org, linux-kernel@vger.kernel.org,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Will Deacon <will.deacon@arm.com>, torvalds@linux-foundation.org,
        boqun.feng@gmail.com
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()
Message-ID: <20151112180003.GE17308@twins.programming.kicks-ass.net>
References: <20151112123123.GZ17308@twins.programming.kicks-ass.net>
 <5644D08D.4080206@caviumnetworks.com>
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On Thu, Nov 12, 2015 at 09:46:53AM -0800, David Daney wrote:

> For CONFIG_CPU_CAVIUM_OCTEON the proper thing would be:
> 
>     smp_wmb();
>     smp_rmb();
> 
> Which expands to exactly the same thing as wmb() because smp_rmb() expands
> to nothing.

OK, so the current code isn't broken because for Cavium wmb is suffient
because rmb is a no-op, and for !Cavium wmb expands to SYNC.

> You yourself seem to have added smp_store_release(), so we could even do:
> 
>      smp_store_release(&lock->h.serving_now, lock->h.serving_now + 1);
> 
> That would leave us to cook up a proper definition of smp_store_release().

That is indeed the better solution.