Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE

Christian Borntraeger
2017-03-03T03:13:52Z

On 03/02/2017 06:55 PM, Arnd Bergmann wrote:
> On Thu, Mar 2, 2017 at 5:51 PM, Christian Borntraeger
> <borntraeger@de.ibm.com> wrote:
>> On 03/02/2017 05:38 PM, Arnd Bergmann wrote:
>>>
>>> This attempts a rewrite of the two macros, using a simpler implementation
>>> for the most common case of having a naturally aligned 1, 2, 4, or (on
>>> 64-bit architectures) 8 byte object that can be accessed with a single
>>> instruction. For these, we go back to a volatile pointer dereference
>>> that we had with the ACCESS_ONCE macro.
>>
>> We had changed that back then because gcc 4.6 and 4.7 had a bug that could
>> removed the volatile statement on aggregate types like the following one
>>
>> union ipte_control {
>> unsigned long val;
>> struct {
>> unsigned long k : 1;
>> unsigned long kh : 31;
>> unsigned long kg : 32;
>> };
>> };
>>
>> See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145
>>
>> If I see that right, your __ALIGNED_WORD(x)
>> macro would say that for above structure sizeof(x) == sizeof(long)) is true,
>> so it would fall back to the old volatile cast and might reintroduce the
>> old compiler bug?

Oh dear, I should double check my sentences in emails before sending...anyway
the full story is referenced in

commit 60815cf2e05057db5b78e398d9734c493560b11e
Merge tag 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/borntraeger/linux
which has a pointer to
http://marc.info/?i=54611D86.4040306%40de.ibm.com
which contains the full story.

>
> Ah, right, that's the missing piece. For some reason I didn't find
> the reference in the source or the git log.
>
>> Could you maybe you fence your simple macro for anything older than 4.9? After
>> all there was no kasan support anyway on these older gcc version.
>
> Yes, that should work, thanks!