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[209.85.218.53]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a90612b33d9sm1238136366b.114.2024.09.23.09.28.48 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Sep 2024 09:28:48 -0700 (PDT) Received: by mail-ej1-f53.google.com with SMTP id a640c23a62f3a-a90188ae58eso570998666b.1 for ; Mon, 23 Sep 2024 09:28:48 -0700 (PDT) X-Forwarded-Encrypted: i=1; AJvYcCUNYqiC4gBmgPGzCPNvZwvNeZ1SVTlehXvSS2EYFAAegXW270wU6HrtWU5AXcuN34lObU16067bQQ==@kvack.org X-Received: by 2002:a17:907:25c2:b0:a8a:780f:4faf with SMTP id a640c23a62f3a-a90d5925392mr1037193466b.47.1727108928289; Mon, 23 Sep 2024 09:28:48 -0700 (PDT) MIME-Version: 1.0 References: <20240912-seq_optimize-v3-1-8ee25e04dffa@gentwo.org> <20240917071246.GA27290@willie-the-truck> <4b546151-d5e1-22a3-a6d5-167a82c5724d@gentwo.org> In-Reply-To: From: Linus Torvalds Date: Mon, 23 Sep 2024 09:28:31 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load acquire To: "Christoph Lameter (Ampere)" Cc: Will Deacon , Thomas Gleixner , Catalin Marinas , Peter Zijlstra , Ingo Molnar , Waiman Long , Boqun Feng , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: C671880013 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 8bda65gctrwxa1timy1qgf8zndrasqk5 X-HE-Tag: 1727108933-952257 X-HE-Meta: U2FsdGVkX1/a+0KjA49LBQQ06o9lqRaoaUJZJb+ykNWfCD1rJe1DuJPyGvaKd494DX7ZoeObaGsP/DYDQHZC0v6gB1EUz6Hg1dykObHPWV5GPD8I7jYy+dPeqZauBORExtPiyISQmqV08nlK8tUnX/GzqlKKBIbnSsFbwQAjeuEqOhNylTA6VGVM682hKqkdaF4a0PjjMT0NgLTQ7k09mrhsEbCMoNnexcWSly+b2yPdhISHdFwb5/bml9mj3doDd8XMuPvlIkECGLjnFmWX5HeOixClWiOHsuFxWbwpo9bcLe5SuIwlwyLx5pfAYerAEeVjpeGdPwhNeOjRKd3s4WGDLSs0qVQwcn/F9nqehObHJHYu+u1ocejA6G8EHqNaTrfSP4Gy1lYvBODSvWc8/slqcl41OM28OoC5Jf+pODzYarb0/HCEGH2dHpULarLXaxsmnH9FHL7V17NrXoRZXuKhXSDwln4eg7erk2mrDY+KurhJTT1gjA22ec8j/CGzQKcp62Yy1YZeMLj0BUSIE82YViUyIfx6LvTe8MS2iCFyVB2NCNXL9A++mFSDCTkteC8rQlK9yhinalI5tUcXxPwqiFJJP5fEi7cFN2UkquknBehEEURdJJNgpCKRU7c86Mu+9ebvMiIpcZUdrvEAADkyV3gUHepF2LgyBYpsiMN/EKIpMIPQqrqVP4gz7IqBrctjPkxVTqsvV8oZPClMPmVABKZnLHQdJ4Ghp97EkqrePznH6CBeiUtu8DUOpy+x/jUnag3Kst3+g7+3DioKrwHCEuXgtzJq6AdPS7oLWSQxU85PKTg+O6oby1ct1hCLnWVfyAz0h2ETAzGps3VrAApeMipST6lt3hNU8Z4S6V2ZVYGSSA3ojKDl4joW1APk8+7UwFbvz77TCzzRiEg07rhLIHtm7M9KvUEYSfVcExJcFgXpqrhrrUGAchwxe1SCKVt10i/5xR581v/jjoX zXk5jDRH xZIreADcTUWW1Q8fV6Tez2M92M7IuQyVmawLKx4u0J/NfaZHZkdZiZzIr70ongY50s2HaxDcwUKYF3+9LFG6afZZ/53wMoTBagGm3jJ8Rz/Z2YmCekT01ED1RkI4JErBtPwpK7Y39/IG2BV2bmHAgKGYHjkU9q4GuH5+2yCzPzcSQWRAMWY2OzHY+fioU3rDVA0MIU/aXXC6Tl8hRHRGcONi9j74KEOOLnclOBrZTDAs+3MA7+PS+kc6jYB3MgrC2agyGjTGukP6FY1HM5o+kc95wEz1zpByqf/M2ZHUVQ2JoiNKaq2wCWgKeeF2DbVAgLp46dz4Jk9thp2jxeYkZfcgnban1EbUgZp5Hr2xhOqX+1Wc3J8vWkElVzucSGYUsIZ1OSB4zzVG5s02/sTwGMA3vQkcRAy8JrZZJpHdLNo9m5iUDFBvlEfXN9HrHki6m2hFEXEw7IfyqJkpKRzUuyJCQgPGXBDSITTg4cRafALRK5/oSfa6fXvLHGzE1mN+WVkjS7EaysGx8KW9FfY7tlRSEiCFFtRvuprbZOEOruIF2GpoDMSI8b3GyJmDbver+/WsgW2D008Kv2s6fWtkJVTy5v33np1Yx3JLhHXFSlnYZo/ILQmKKfKE0s+8bhtDLEb5U5lX1e8UrBLkuPlKYitvMlyV3J1dco5MQVoPkzz0A5ZpetXSGZaJIUCh4PCyCErElaD0NdTJEokAAygBzXsXRFh/iK9qj1D+49hyAHa/wDB8= 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 Wed, 18 Sept 2024 at 08:22, Linus Torvalds wrote: > > On Wed, 18 Sept 2024 at 13:15, Christoph Lameter (Ampere) wrote: > > > > Other arches do not have acquire / release and will create additional > > barriers in the fallback implementation of smp_load_acquire. So it needs > > to be an arch config option. > > Actually, I looked at a few cases, and it doesn't really seem to be true. Bah. I ended up just committing the minimal version of this all. I gave Christoph credit for the commit, because I stole his commit message, and he did most of the work, I just ended up going "simplify, simplify, simplify". I doubt anybody will notice, and smp_load_acquire() is the future. Any architecture that does badly on it just doesn't matter (and, as mentioned, I don't think they even exist - "smp_rmb()" is generally at least as expensive). Linus