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

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Message-ID: <20150115200119.GA29684@redhat.com>
Date:	Thu, 15 Jan 2015 21:01:19 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.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

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?

Oleg.

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