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From: "Huang, Ying" <ying.huang@intel.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>, Andrew Morton
 <akpm@linux-foundation.org>,  David Hildenbrand <david@redhat.com>,
  Matthew Wilcox <willy@infradead.org>,  Gao Xiang <xiang@kernel.org>,  Yu
 Zhao <yuzhao@google.com>,  Yang Shi <shy828301@gmail.com>,  Michal Hocko
 <mhocko@suse.com>,  Kefeng Wang <wangkefeng.wang@huawei.com>,  Barry Song
 <21cnbao@gmail.com>,  Chris Li <chrisl@kernel.org>,  <linux-mm@kvack.org>,
  <linux-kernel@vger.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)
In-Reply-To: <b22a222b-7fd8-4648-84a7-21d35f529f27@arm.com> (Ryan Roberts's
	message of "Thu, 21 Mar 2024 12:21:46 +0000")
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	<20240311150058.1122862-5-ryan.roberts@arm.com>
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	<d6ac1097-2ca3-4e6d-902d-1b942cacf0fb@arm.com>
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	<b22a222b-7fd8-4648-84a7-21d35f529f27@arm.com>
Date: Fri, 22 Mar 2024 10:38:20 +0800
Message-ID: <87r0g3q9cz.fsf_-_@yhuang6-desk2.ccr.corp.intel.com>
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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