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[209.85.208.49]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-5bebc081dc6sm5701474a12.91.2024.08.19.09.26.02 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 Aug 2024 09:26:02 -0700 (PDT) Received: by mail-ed1-f49.google.com with SMTP id 4fb4d7f45d1cf-5befe420fc2so1397522a12.3 for ; Mon, 19 Aug 2024 09:26:02 -0700 (PDT) X-Forwarded-Encrypted: i=1; AJvYcCW9lEWg7Y4h7SrYs2Lf7U6I/IL2XdB4ABwUrbYKWafWnEzmckVMVxCGGDH4MnFBNSXxoI3Pb5EM8zv3owSI+DQAb+8= X-Received: by 2002:a05:6402:254e:b0:5be:daa6:9139 with SMTP id 4fb4d7f45d1cf-5bedb6f274cmr4310212a12.20.1724084762085; Mon, 19 Aug 2024 09:26:02 -0700 (PDT) MIME-Version: 1.0 References: <20240813-seq_optimize-v1-1-84d57182e6a7@gentwo.org> In-Reply-To: From: Linus Torvalds Date: Mon, 19 Aug 2024 09:25:45 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH RFC] Avoid memory barrier in read_seqcount() through load acquire To: Mark Rutland Cc: cl@gentwo.org, Catalin Marinas , Will Deacon , Peter Zijlstra , Ingo Molnar , Waiman Long , Boqun Feng , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: 33i8bzfyniy4jdihy5opjs7dby1rijud X-Rspamd-Queue-Id: CBAF9180026 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1724084765-63170 X-HE-Meta: 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 hVwiYABp 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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: On Mon, 19 Aug 2024 at 01:46, Mark Rutland wrote: > > If you cannot disclose that for some reason, just say "on my ARM64 test > machine" or something like that, so that we're not implying that this is > true for all ARM64 implementations. It's the same machine I have - an Ampere Altra. It's a standard Neoverse N1 core, afaik. It might also be a good idea to just point to the ARM documentation, although I don't know how stable those web addresses are: https://developer.arm.com/documentation/102336/0100/Load-Acquire-and-Store-Release-instructions and quoting the relevant part on that page: "Weaker ordering requirements that are imposed by Load-Acquire and Store-Release instructions allow for micro-architectural optimizations, which could reduce some of the performance impacts that are otherwise imposed by an explicit memory barrier. If the ordering requirement is satisfied using either a Load-Acquire or Store-Release, then it would be preferable to use these instructions instead of a DMB" where that last sentence is basically ARM saying that load-acquire is better than load+DMB and should be preferred. Linus