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[209.85.208.45]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-5bd187f4fe2sm3161309a12.9.2024.08.13.13.02.06 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 13 Aug 2024 13:02:06 -0700 (PDT) Received: by mail-ed1-f45.google.com with SMTP id 4fb4d7f45d1cf-5bba25c1e15so7049323a12.2 for ; Tue, 13 Aug 2024 13:02:06 -0700 (PDT) X-Forwarded-Encrypted: i=1; AJvYcCW4ywqHDZLwxz4zbUrgWe6LTeN47jf5fpVTIExezNGYUGRAHHCHmdmZ/yy9a0GW/QKmnZDWZ3XowjMVi2T9RYmBHCE= X-Received: by 2002:a05:6402:278e:b0:5a3:5218:5d82 with SMTP id 4fb4d7f45d1cf-5bea1c7b117mr401494a12.18.1723579326438; Tue, 13 Aug 2024 13:02:06 -0700 (PDT) MIME-Version: 1.0 References: <20240813-seq_optimize-v1-1-84d57182e6a7@gentwo.org> <183ee6fa-1d42-4a01-8446-4f20942680d2@redhat.com> <5d54a189-3c2b-440a-9626-4e00e95a7f77@redhat.com> In-Reply-To: <5d54a189-3c2b-440a-9626-4e00e95a7f77@redhat.com> From: Linus Torvalds Date: Tue, 13 Aug 2024 13:01:50 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH RFC] Avoid memory barrier in read_seqcount() through load acquire To: Waiman Long Cc: cl@gentwo.org, Catalin Marinas , Will Deacon , Peter Zijlstra , Ingo Molnar , 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-Rspam-User: X-Rspamd-Queue-Id: B507014002D X-Rspamd-Server: rspam01 X-Stat-Signature: 7sida1yfapxhpqt4o57xzm9ksnto9ehd X-HE-Tag: 1723579329-19642 X-HE-Meta: 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 vSTpAAH3 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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 Tue, 13 Aug 2024 at 12:58, Waiman Long wrote: > > Sorry for the confusion. What you said above is actually the reason that > I ask this question. In the same way, smp_rmb()/wmb() is available for > all arches. I am actually asking if it should be a flag that indicates > the arch's preference to use acquire/release over rmb/wmb. I think that if an arch says it has native acquire/release, we should basically assume that it's the better model. I mean, we could certainly use "PREFERS" instead of "HAS", but is there any real reason to do that? Do we suddenly expect that people would make a CPU that has native acquire/release, and it would somehow then prefer a full read barrier? Linus