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

[Alexei Starovoitov]

On Mon, Nov 24, 2014 at 12:34 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Nov 24, 2014 at 12:04 PM, David Howells <dhowells@xxxxxxxxxx> 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.

I've changed gcc pr58145-1.c reproducer to use
__read_once_size() approach above, but it
didn't help to workaround the bug.
gcc 'sra' pass still lost 'volatile' modifier.

modified reproducer:
struct S { unsigned int data; };
void bar(int val)
{
  struct S _s = { .data = val };
  *(volatile struct S *) 0x880000UL = ACCESS_ONCE(&_s);
}
assembler code looks as expected, but
internal gcc state after 'sra' pass is
missing volatile...
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