linux-kernel - 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: Fri, 22 Mar 2024 10:38:20 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Ryan Roberts <ryan.roberts@....com>, 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: Can you help us on memory barrier usage? (was Re: [PATCH v4 4/6]
 mm: swap: Allow storage of all mTHP orders)

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.
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