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?
Linus
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