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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