Date: Thu, 12 Nov 2015 06:32:31 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: ralf@linux-mips.org, ddaney@caviumnetworks.com,
        linux-kernel@vger.kernel.org, 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: <20151112143231.GS3972@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20151112123123.GZ17308@twins.programming.kicks-ass.net>
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On Thu, Nov 12, 2015 at 01:31:23PM +0100, Peter Zijlstra wrote:
> Hi
> 
> I think the MIPS arch_spin_unlock() is borken.
> 
> spin_unlock() must have RELEASE semantics, these require that no LOADs
> nor STOREs leak out from the critical section.
> 
> >From what I know MIPS has a relaxed memory model which allows reads to
> pass stores, and as implemented arch_spin_unlock() only issues a wmb
> which doesn't order prior reads vs later stores.
> 
> Therefore upgrade the wmb() to smp_mb().
> 
> (Also, why the unconditional wmb, as opposed to smp_wmb() ?)

One guess is that they want to order I/O accesses within the critical
section?

							Thanx, Paul

> Maybe-Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> diff --git a/arch/mips/include/asm/spinlock.h b/arch/mips/include/asm/spinlock.h
> index 40196bebe849..b2ca13f06152 100644
> --- a/arch/mips/include/asm/spinlock.h
> +++ b/arch/mips/include/asm/spinlock.h
> @@ -140,7 +140,7 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
>  static inline void arch_spin_unlock(arch_spinlock_t *lock)
>  {
>  	unsigned int serving_now = lock->h.serving_now + 1;
> -	wmb();
> +	smp_mb();
>  	lock->h.serving_now = (u16)serving_now;
>  	nudge_writes();
>  }
> 

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