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

[Christian Borntraeger <borntraeger@xxxxxxxxxx>]

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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