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From: Roman Gushchin To: Muchun Song Cc: Shakeel Butt , Yue Zhao , Linux Memory Management List , Andrew Morton , Johannes Weiner , Michal Hocko , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations Message-ID: References: <20230220230624.lkobqeagycx7bi7p@google.com> <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev> <61CA5FFC-547B-4F56-9238-C9096223BF80@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <61CA5FFC-547B-4F56-9238-C9096223BF80@linux.dev> X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 8050420022 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: xujdzj454ekxy658t1rbjusz33sntdws X-HE-Tag: 1677001756-941660 X-HE-Meta: U2FsdGVkX196teqY/2YDahuri1oGvm13+zjr94xw+Eh55aoFcagtd9t38xx01c8HIEmmn4jBThUag7OUqjOC3iCzLU0UFrhjGDPrF0loYOVJDs0rlog+rDQhrNwRvd21Anxsn60rXHVlAvvHGqBQ3jcmC8MM7cmT7KKEa1TCvGFOuaMM/dcoJm9o7vJ0UhM1WM0EWau3/X3JCkAQn8f41PCaFsL6Xv6qTfFNMC6ytfZIr1GCOSkJe101a4kcqtr09AwaONn9re3X2+SowpEIgwz/AfVbWBdEjorZBYXPu7jOnGJdOpU5aqPLNvF1vcoDTAFWWu2FIIDpiSkKwwsZt+9YTFgEbqK65pNUiYeD4XvBnKLfATwdxybay5WaG6wEkZ8gWWXPsWCHAZeaDHgaZvO+dJDMHFgaimGhJXJpJOpmj/AmZUltXx6hyAwybKrd4Rqk5EGzbKfQhPTfBAsu+5OIMTLLsPmKSZZTvqOlQAga3RIFFpOxpriKsesB+xlcmpScTGN0InCTgAkmMjDvJiPj+9txNIyRQixZ+n/HTcjonM1NH/u09qhx3hWADcFeoPcIgDOSSQGuj07td3R/8m4KIpfNXB/eH/IcmmHR1u5wei7kHx19K/Gu8byStn0B+gySkoOsR1ddwbTV6l5Nhlk2s386YomFLEY1wuq9dc4SLVCTn0/fscDTB3mBw4aZBAExacjcxsnfvImUOoFh5lxbUKxeDRw3HbBA+jU7Ocksgji43w3QWbpyDn2m/VFaH5B5LcufsRi98uw0a9mS5KuuIsGp9kcKoAE830z5/BPgUxByVzX3p3o8EUnkiii5JmxngRFjL8ii1wv93tfopu4F4vqFlGSEez4erQ1IGeWnKUoKFHKzihwgp/2YJH9H6+bBfFYyID8HPNJ+jddqYAhoDfssyuGPoyOIE6TiPZihHjKR+eSKs5aRw0/M62EH40tx1n0sqQB2CU108Jv +Icu6s7N gl0aWoaDn4QMGBarCjA6vIj1q0Cq8jmf+np9FwAM8Wai4Y9akseImgd9cuY9aUhXiYy8/1hSIbQFt0SIMhNeogR00vAL+lwkNXwqHaV86fbdWdTP5AjFu8MuUYNZjwljYr4SXFKsgUU+iVHLhbR7sXGZqozoRE0X7r+ELABUFONhWfDpTgpQ0IfT02Oms4CrGxZeQuraJZVphxrJZNx6jak0yp4YMonCEtSZQyEcKz1Cyo9JdGXDzB0r6QBfBL3VNpPK6 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: On Tue, Feb 21, 2023 at 03:22:32PM +0800, Muchun Song wrote: > > > > On Feb 21, 2023, at 13:17, Roman Gushchin wrote: > > > >> On Feb 20, 2023, at 3:06 PM, Shakeel Butt wrote: > >> > >> On Mon, Feb 20, 2023 at 01:09:44PM -0800, Roman Gushchin wrote: > >>>> On Mon, Feb 20, 2023 at 11:16:38PM +0800, Yue Zhao wrote: > >>>> The knob for cgroup v2 memory controller: memory.oom.group > >>>> will be read and written simultaneously by user space > >>>> programs, thus we'd better change memcg->oom_group access > >>>> with atomic operations to avoid concurrency problems. > >>>> > >>>> Signed-off-by: Yue Zhao > >>> > >>> Hi Yue! > >>> > >>> I'm curious, have any seen any real issues which your patch is solving? > >>> Can you, please, provide a bit more details. > >>> > >> > >> IMHO such details are not needed. oom_group is being accessed > >> concurrently and one of them can be a write access. At least > >> READ_ONCE/WRITE_ONCE is needed here. > > > > Needed for what? > > > > I mean it’s obviously not a big deal to put READ_ONCE()/WRITE_ONCE() here, but I struggle to imagine a scenario when it will make any difference. IMHO it’s easier to justify a proper atomic operation here, even if it’s most likely an overkill. > > > > My question is very simple: the commit log mentions “… to avoid concurrency problems”, so I wonder what problems are these. > > I think there is no difference in the assembly code between them in most > cases. The only intention that I can think of is to avoid the potential > complaint (data race) emitted by KCSAN. +1 And it might be a totally good reason for this change, let's just make it clear, instead of pretending to fix non-existing concurrency problems. Thanks!