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]

Ryan Roberts <ryan.roberts@xxxxxxx> writes:

> On 22/03/2024 02:38, 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.
>
> For the situation we are discussing, writer() is always called with a spin lock
> held. So spin_unlock() will act as the barrier in this case; that's my argument
> for not needing the explicit barrier(), anyway. Happy to be told I'm wrong.

Yes.  spin_unlock() is a barrier too.  There are some operations between
writer() and spin_unlock(), so I want to check whether it make any sense
to add a barrier earlier.

>> It's acceptable for reader() to read the written value a little later.
>> Our questions are,
>> 
>> 1. because it's impossible for accessing "unsigned char" to cause
>> tearing.  So, WRITE_ONCE()/READ_ONCE()/barrier() isn't necessary for
>> correctness, right?
>> 
>> 2. we use barrier() and READ_ONCE() in writer() and reader(), because we
>> want to make writing take effect ASAP.  Is it a good practice?  Or it's
>> a micro-optimization that should be avoided?

--
Best Regards,
Huang, Ying