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

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Date: Mon, 25 Mar 2024 11:20:29 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,  Andrew Morton
 <akpm@...ux-foundation.org>,  David Hildenbrand <david@...hat.com>,
  Matthew Wilcox <willy@...radead.org>,  Gao Xiang <xiang@...nel.org>,  Yu
 Zhao <yuzhao@...gle.com>,  Yang Shi <shy828301@...il.com>,  Michal Hocko
 <mhocko@...e.com>,  Kefeng Wang <wangkefeng.wang@...wei.com>,  Barry Song
 <21cnbao@...il.com>,  Chris Li <chrisl@...nel.org>,  <linux-mm@...ck.org>,
  <linux-kernel@...r.kernel.org>
Subject: 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@....com> 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