linux-kernel - Re: [PATCH 4/8] x86/spinlock: Leftover conversion ACCESS_ONCE->READ

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Message-ID: <54B82A8B.7000809@de.ibm.com>
Date:	Thu, 15 Jan 2015 22:00:59 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
	xen-devel@...ts.xenproject.org, linux-mm@...ck.org
Subject: Re: [PATCH 4/8] x86/spinlock: Leftover conversion ACCESS_ONCE->READ_ONCE

Am 15.01.2015 um 21:01 schrieb Oleg Nesterov:
> On 01/15, Christian Borntraeger wrote:
>>
>> Am 15.01.2015 um 20:38 schrieb Oleg Nesterov:
>>> On 01/15, Christian Borntraeger wrote:
>>>>
>>>> --- a/arch/x86/include/asm/spinlock.h
>>>> +++ b/arch/x86/include/asm/spinlock.h
>>>> @@ -186,7 +186,7 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
>>>>  	__ticket_t head = ACCESS_ONCE(lock->tickets.head);
>>>>
>>>>  	for (;;) {
>>>> -		struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
>>>> +		struct __raw_tickets tmp = READ_ONCE(lock->tickets);
>>>
>>> Agreed, but what about another ACCESS_ONCE() above?
>>>
>>> Oleg.
>>
>> tickets.head is a scalar type, so ACCESS_ONCE does work fine with gcc 4.6/4.7.
>> My goal was to convert all accesses on non-scalar types
> 
> I understand, but READ_ONCE(lock->tickets.head) looks better anyway and
> arch_spin_lock() already use READ_ONCE() for this.
> 
> So why we should keep the last ACCESS_ONCE() in spinlock.h ? Just to make
> another cosmetic cleanup which touches the same function later?

OK, I will change that one as well.


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