Re: Can you help us on memory barrier usage? (was Re: [PATCH v4 4/6] mm: swap: Allow storage of all mTHP orders)

[Huang, Ying]

"Paul E. McKenney" <paulmck@xxxxxxxxxx> writes:

> On Sat, Mar 23, 2024 at 11:11:09AM +0900, Akira Yokosawa wrote:
>> [Use Paul's reachable address in CC;
>>  trimmed CC list, keeping only those who have responded so far.]
>> 
>> Hello Huang,
>> Let me chime in.
>> 
>> On Fri, 22 Mar 2024 06:19:52 -0700, Huang, Ying wrote:
>> > Hi, Paul,
>> > 
>> > Can you help us on WRITE_ONCE()/READ_ONCE()/barrier() usage as follows?
>> > For some example kernel code as follows,
>> > 
>> > "
>> > unsigned char x[16];
>> > 
>> > void writer(void)
>> > {
>> >         memset(x, 1, sizeof(x));
>> >         /* To make memset() take effect ASAP */
>> >         barrier();
>> > }
>> > 
>> > unsigned char reader(int n)
>> > {
>> >         return READ_ONCE(x[n]);
>> > }
>> > "
>> > 
>> > where, writer() and reader() may be called on 2 CPUs without any lock.
>> > It's acceptable for reader() to read the written value a little later.
>
> What are your consistency requirements?  For but one example, if reader(3)
> gives the new value, is it OK for a later call to reader(2) to give the
> old value?

writer() will be called with a lock held (sorry, my previous words
aren't correct here).  After the racy checking in reader(), we will
acquire the lock and check "x[n]" again to confirm.  And, there are no
dependencies between different "n".  All in all, we can accept almost
all races between writer() and reader().

My question is, if there are some operations between writer() and
unlocking in its caller, whether does barrier() in writer() make any
sense?  Make write instructions appear a little earlier in compiled
code?  Mark the memory may be read racy?  Or doesn't make sense at all?

> Until we know what your requirements are, it is hard to say whether the
> above code meets those requirements.  In the meantime, I can imagine
> requirements that it meets and others that it does not.
>
> Also, Akira's points below are quite important.

Replied for his email.

--
Best Regards,
Huang, Ying