Re: [PATCH] locking/atomics: don't alias ____ptr

[Dmitry Vyukov]

On Wed, Jun 28, 2017 at 2:24 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Wed, 28 Jun 2017, Dmitry Vyukov wrote:
>> On Wed, Jun 28, 2017 at 1:10 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> >  #define cmpxchg_local(ptr, old, new)                   \
>> >  ({                                                     \
>> > -       __typeof__(ptr) ____ptr = (ptr);                \
>> > -       kasan_check_write(____ptr, sizeof(*____ptr));   \
>> > -       arch_cmpxchg_local(____ptr, (old), (new));      \
>> > +       kasan_check_write((ptr), sizeof(*(ptr)));       \
>> > +       arch_cmpxchg_local((ptr), (old), (new));        \
>>
>>
>> /\/\/\/\/\/\/\/\/\/\/\/\
>>
>> These are macros.
>> If ptr is foo(), then we will call foo() twice.
>
> If that's true, the foo() will be evaluated a gazillion more times down the
> way to the end of this macro maze.

No. If we do:

__typeof__(ptr) ____ptr = (ptr);

and then only use ____ptr, then ptr is evaluated only once regardless
of what the rest of macros do.