Re: [PATCH glibc 5/9] glibc: Perform rseq(2) registration at C startup and thread creation (v17)

[Florian Weimer <fw@deneb.enyo.de>]

* Mathieu Desnoyers:

>>>>> +/* struct rseq is aligned on 4 * 8 bytes to ensure it is always
>>>>> +   contained within a single cache-line.
>>>>> +
>>>>> +   A single struct rseq per thread is allowed.  */
>>>>> +struct rseq
>>>>> +  {
>>>>> +    /* Restartable sequences cpu_id_start field. Updated by the
>>>>> +       kernel. Read by user-space with single-copy atomicity
>>>>> +       semantics. This field should only be read by the thread which
>>>>> +       registered this data structure. Aligned on 32-bit. Always
>>>> 
>>>> What does “Aligned on 32-bit” mean in this context?  Do you mean to
>>>> reference 32-*byte* alignment here?
>>>
>>> No. I really mean 32-bit (4-byte). Being aligned on 32-byte guarantees that
>>> this field is aligned at least on 4-byte. This is required by single-copy
>>> atomicity semantics.
>>>
>>> Should I update this comment to state "Aligned on 4-byte" instead ?
>> 
>> I think this is implied by all Linux ABIs.  And the explicit alignment
>> specification for struct rseq makes the alignment 32 bytes.
>
> Unless a structure ends up being packed, which is of course not the case
> here.
>
> I would prefer to keep the comment about 32-bit alignment requirement on
> the specific fields, because the motivation for alignment requirement is
> much more strict for fields (correctness) than the motivation for alignment
> of the structure (performance).

But the correctness is already enforced by the compiler, so I fail to
see point of mentioning this in the comment.

Anyway, I don't want to make a big deal of it.  Please leave it in if
you think it is ehlpful.

> x32 should not be an issue as explained above, so I'm very open to
> add this "uptr" for user-space only.

Okay, then please use anonymous unions and structs as necessary, to
ensure that the uptr field can be reached on all platforms in the same
way.