Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Thu, Jan 4, 2018 at 2:20 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> #define array_access(base, idx, max) ({                         \
>         union { typeof(base[0]) _val; unsigned long _bit; } __u;\
>         unsigned long _i = (idx);                               \
>         unsigned long _m = (max);                               \
>         unsigned long _mask = _i < _m ? ~0 : 0;                 \
>         OPTIMIZER_HIDE_VAR(_mask);                              \
>         __u._val = base[_i & _mask];                            \
>         __u._bit &= _mask;                                      \
>         __u._val; })

That

    __u._val = base[_i & _mask];

thing would have to be READ_ONCE() to make it all ok for the special
cases without locking etc.

              Linus