Akira Yokosawa wrote:
> Daniel Xu wrote:
>> Hi Akira,
>>
>> Thanks for taking a look!
>>
>> On Tue, Dec 17, 2024, at 5:53 PM, Akira Yokosawa wrote:
>>> Hello,
>>>
>>> Daniel Xu wrote:
>>>> WRITE_ONCE() is needed here to prevent store tears and other unwanted
>>>> compiler optimizations.
>>>
>>> That might be true if there were chances of these two accesses to
>>> race with each other.
>>>
>>> I don't see any possibility of such races.
>>>
>>> Can you elaborate?
>>
>> My understanding is that read_seqbegin() and read_seqretry()
>> can execute at any time. That means the read side access of
>> the sequence number can occur doing an increment.
>>
>> To prevent the reader from reading a partially written value,
>> we need the WRITE_ONCE() to ensure the relaxed atomic
>> write.
>
> OK, now I see the race there.
>
> But see below.
>
[...]
>
> Those two READ_ONCE()s don't have any concurrent writes.
> So there seems to be no need of them, IIUC.
>
Furthermore, in sequence locking, readers will retry if they see
an inconsistent sequence count due to store tearing, so WRITE_ONCE()
is not necessary, either.
READ_ONCE()s in read_seqbegin() and read_seqretry() are there for
preventing read accesses in retry loops from being optimized out.
Thanks, Akira
