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

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Message-ID: <54B81A37.80109@de.ibm.com>
Date:	Thu, 15 Jan 2015 20:51:19 +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 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 as until 
"kernel: tighten rules for ACCESS ONCE" is merged because anything else would be
a Whac-a-mole like adventure (I learned that during the last round in next: all
conversions in this series fix up changes made during this merge window)

We probably going to do a bigger bunch of bulk conversion later on when 
"kernel: tighten rules for ACCESS ONCE" prevents new problems.

Makes sense?

Christian

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