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Tue, 13 Aug 2024 19:58:28 +0000 (UTC) Message-ID: <5d54a189-3c2b-440a-9626-4e00e95a7f77@redhat.com> Date: Tue, 13 Aug 2024 15:58:27 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] Avoid memory barrier in read_seqcount() through load acquire To: Linus Torvalds 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 References: <20240813-seq_optimize-v1-1-84d57182e6a7@gentwo.org> <183ee6fa-1d42-4a01-8446-4f20942680d2@redhat.com> Content-Language: en-US From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 X-Stat-Signature: ecur4pb7z38z5671cfo738wp6sm6gzdh X-Rspamd-Queue-Id: A51FF1C0009 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1723579116-127891 X-HE-Meta: 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 f3Cn2/a4 xOzr8nrw8JKDgcCZ1ZKfl1nTgVyxdU/Mo73ZDZt2McqVVYWq7wHgMfCg81YWoyZzSRJO4lcdqJQ3c16xRcyuW7W3/0ueF/zEP3TJgPLEsQa2BmWhaHD4wAeBrQZ7clI5eGdQMzeLgGdKvmy/y1qbtFxeqnPzf+09EqKmiIU4iWdHwvdyrYO9G8qu3fj/nknESmzWgXuNAN6NGWkNBRBCmIUohYFGQcQmlfco0Om2sNzfStOxk+t4E82shLmLS1ggUa6U21VaM1joLOKsG/Gv/prDkKlB/rrFDbCZHLNhP5JVf6IxYHjjszOj+vuwcR7KwzjZGkbgfx3Dd0Rs0rmX108cQ5uHWNZVp3S2WiVGrcdpnHF9htTzbCFZtYmyV3p69+bpzg0Egwhat4BIZ0XbiTjQ/m4Kkvpeob1akNqBDCeX3hrwFOl0XRQ1oHWWQ6oRKgoCTfv5JbHeb6po= 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 8/13/24 15:48, Linus Torvalds wrote: > On Tue, 13 Aug 2024 at 12:01, Waiman Long wrote: >> Do we need a new ARCH flag? > I'm confused by that question. > > That's clearly exactly what that ARCH_HAS_ACQUIRE_RELEASE is. > > Obviously all architectures "have" it - in the sense that we always > have access to a "smp_load_acquire()/smp_store_release()". 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. Cheers, Longman > > But if the architecture doesn't support it natively, the old rmb/wmb > model may be preferred. > > Although maybe we're at the point where we don't even care about that. > > Linus >