Messages in this thread | | | Date | Mon, 24 Nov 2014 13:02:40 -0800 | Subject | Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types | From | Linus Torvalds <> |
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On Mon, Nov 24, 2014 at 12:53 PM, Christian Borntraeger <borntraeger@de.ibm.com> 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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