Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types

[Christian Borntraeger]

Am 24.11.2014 um 22:02 schrieb Linus Torvalds:
> On Mon, Nov 24, 2014 at 12:53 PM, Christian Borntraeger
> <borntraeger@xxxxxxxxxx> wrote:
>>
>> That looks like a lot of changes all over ACCESS_ONCE -> ASSIGN_ONCE:
>> git grep "ACCESS_ONCE.*=.*"
>> gives me 200 placea not in Documentation.
> 
> Yeah, that's a bit annoying.
> 
> How about a combination of the two:
> 
>  - accept the fact that right now ACCESS_ONCE() is fairly widespread
> (even for writing)
> 
>  - but also admit that we'd be better off with a nicer interface
> 
> and make the solution be:
> 
>  - make ACCESS_ONCE() only work on scalars, and deprecate it
> 
>  - add new "read_once()" and "write_once()" interfaces that *do* work
> on (appropriately sized) structures and unions, and start migrating
> things over. In particular, start with the ones that can no longer use
> ACCESS_ONCE() because they aren't scalar..
> 
> That second point would make the conversion patches actually easier to
> read. Instead of this:
> 
>  static inline int arch_spin_is_locked(arch_spinlock_t *lock)
>  {
> -       struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
> +       arch_spinlock_t tmp = {};
> 
> -       return tmp.tail != tmp.head;
> +       tmp.head_tail =ACCESS_ONCE(lock->head_tail);
> +       return tmp.tickets.tail != tmp.tickets.head;
>  }
> 
> which isn't *complex*, but is also not an obvious conversion, we'd have just
> 
>  static inline int arch_spin_is_locked(arch_spinlock_t *lock)
>  {
> -       struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
> -       struct __raw_tickets tmp = read_once(lock->tickets);
> 
>         return tmp.tail != tmp.head;
>  }
> 
> which is a much simpler and more obvious change.
> 
> And then we could slowly try to migrate existing ACCESS_ONCE() users
> over (particularly writers).
> 
> Hmm? Too much?

I will give it a try. I will start with Alexei's version for ACCESS_ONCE and your snippets to build read_once and write_once. The only open question is, what to do with the "too large" accesses. Pauls initial patch showed several 
places, e.g. kernel/sched/fair.c accessing an u64 even on 32bit:
[...]
   age_stamp = ACCESS_ONCE(rq->age_stamp);
        avg = ACCESS_ONCE(rq->rt_avg);
[...]

I think I will simply not touch those...



Christian

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