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McKenney" Cc: Ryan Roberts , Andrew Morton , David Hildenbrand , Matthew Wilcox , Gao Xiang , Yu Zhao , Yang Shi , Michal Hocko , Kefeng Wang , Barry Song <21cnbao@gmail.com>, Chris Li , , 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: (Ryan Roberts's message of "Thu, 21 Mar 2024 12:21:46 +0000") References: <20240311150058.1122862-1-ryan.roberts@arm.com> <20240311150058.1122862-5-ryan.roberts@arm.com> <87jzm751n3.fsf@yhuang6-desk2.ccr.corp.intel.com> <8734skryev.fsf@yhuang6-desk2.ccr.corp.intel.com> Date: Fri, 22 Mar 2024 10:38:20 +0800 Message-ID: <87r0g3q9cz.fsf_-_@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 987CB2001C X-Stat-Signature: ygurdnxet1np9oui5gcb7o5wwrng9co7 X-HE-Tag: 1711075219-177003 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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