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

[Ryan Roberts <ryan.roberts@arm.com>]

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.

> 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