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

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 24.11.2014 um 21:34 schrieb Linus Torvalds:
> On Mon, Nov 24, 2014 at 12:04 PM, David Howells <dhowells@redhat.com> wrote:
>>
>> Reserve ACCESS_ONCE() for reading and add an ASSIGN_ONCE() or something like
>> that for writing?
> 
> I wouldn't mind that. We've had situations where reading and writing
> isn't really similar - like alpha where reading a byte is atomic, but
> writing one isn't.
> 
> Then we could also make it have the "get_user()/put_user()" kind of
> semantics - .and then use the same "sizeopf()" tricks that we use for
> get_user/put_user.
> 
> That would actually work around the gcc bug a completely different way:
> 
>   #define ACCESS_ONCE(p) \
>       ({ typeof(*p) __val; __read_once_size(p, &__val, sizeof(__val)); __val; })
> 
> and then we can do things like this:
> 
>   static __always_inline void __read_once_size(volatile void *p, void
> *res, int size)
>   {
>        switch (size) {
>        case 1: *(u8 *)res = *(volatile u8 *)p; break;
>        case 2: *(u16 *)res = *(volatile u16 *)p; break;
>        case 4: *(u32 *)res = *(volatile u32 *)p; break;
> #ifdef CONFIG_64BIT
>        case 8: *(u64 *)res = *(volatile u64 *)p; break;
> #endif
>        }
>   }
> 
> and same for ASSIGN_ONCE(val, p).
> 
> That also hopefully avoids the whole "oops, gcc has a bug", because
> the actual volatile access is always done using a scalar type, even if
> the type of "__val" may in fact be a structure.
> 
> Christian, how painful would that be? Sorry to try to make you do a
> totally different approach..

That looks like a lot of changes all over ACCESS_ONCE -> ASSIGN_ONCE:
git grep "ACCESS_ONCE.*=.*" 
gives me 200 placea not in Documentation.

Then there is still the 64bit accesses on 32bit via ACCESS_ONCE problem, which we could detect with a default cause in your code. We would need to audit and fix all places :-/


So the last proposal from Alexei, seems easier (for me at least :-) )