Re: [Xen-devel] [PATCH 4/8] x86/spinlock: Leftover conversion ACCESS_ONCE->READ_ONCE

[Oleg Nesterov <oleg@xxxxxxxxxx>]

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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