linux-kernel - Re: [RFC][PATCH] mips: Fix arch_spin

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Date:	Thu, 12 Nov 2015 14:50:00 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, ralf@...ux-mips.org,
	ddaney@...iumnetworks.com, linux-kernel@...r.kernel.org,
	Will Deacon <will.deacon@....com>,
	torvalds@...ux-foundation.org, boqun.feng@...il.com
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()

"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:

> 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?

Isn't that what mmiowb() is for?

>> Maybe-Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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();
>>  }
>> 
>

-- 
Måns Rullgård
mans@...sr.com
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