Re: [PATCH] xen/x86: irq: Avoid a TOCTOU race in pirq_spin_lock_irq_desc()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.08.2020 21:25, Julien Grall wrote:
> Hi Andrew,
> 
> Sorry for the late answer.
> 
> On 23/07/2020 14:59, Andrew Cooper wrote:
>> On 23/07/2020 14:22, Julien Grall wrote:
>>> Hi Jan,
>>>
>>> On 23/07/2020 12:23, Jan Beulich wrote:
>>>> On 22.07.2020 18:53, Julien Grall wrote:
>>>>> --- a/xen/arch/x86/irq.c
>>>>> +++ b/xen/arch/x86/irq.c
>>>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>>>          for ( ; ; )
>>>>>        {
>>>>> -        int irq = pirq->arch.irq;
>>>>> +        int irq = read_atomic(&pirq->arch.irq);
>>>>
>>>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>>>> would want this to be ACCESS_ONCE(). So I guess now is the time
>>>> to settle which one to prefer in new code (or which criteria
>>>> there are to prefer one over the other).
>>>
>>> I would prefer if we have a single way to force the compiler to do a
>>> single access (read/write).
>>
>> Unlikely to happen, I'd expect.
>>
>> But I would really like to get rid of (or at least rename)
>> read_atomic()/write_atomic() specifically because they've got nothing to
>> do with atomic_t's and the set of functionality who's namespace they share.
> 
> Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()?

Wouldn't this lead to confusion with Linux'es macros of the same names?

> I would also suggest to move them implementation in a new header asm/lib.h.

Probably.

Jan