Received-SPF: pass (google.com: domain of linux-kernel+bounces-111205-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Message-ID: <44b2cf99-7d87-42ac-8446-fe0822c89c97@arm.com>
Date: Fri, 22 Mar 2024 09:23:44 +0000
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: Can you help us on memory barrier usage? (was Re: [PATCH v4 4/6]
 mm: swap: Allow storage of all mTHP orders)
Content-Language: en-GB
To: "Huang, Ying" <ying.huang@intel.com>,
 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: 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
References: <20240311150058.1122862-1-ryan.roberts@arm.com>
 <20240311150058.1122862-5-ryan.roberts@arm.com>
 <87jzm751n3.fsf@yhuang6-desk2.ccr.corp.intel.com>
 <d6ac1097-2ca3-4e6d-902d-1b942cacf0fb@arm.com>
 <8734skryev.fsf@yhuang6-desk2.ccr.corp.intel.com>
 <b22a222b-7fd8-4648-84a7-21d35f529f27@arm.com>
 <87r0g3q9cz.fsf_-_@yhuang6-desk2.ccr.corp.intel.com>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <87r0g3q9cz.fsf_-_@yhuang6-desk2.ccr.corp.intel.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

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