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Tue, 13 Aug 2024 20:23:54 +0000 (UTC) Message-ID: Date: Tue, 13 Aug 2024 16:23:54 -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> <5d54a189-3c2b-440a-9626-4e00e95a7f77@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 10.30.177.12 X-Rspamd-Queue-Id: E7226A0003 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: xbym6a7wqu9bxrot448rt8zi7okn4omc X-HE-Tag: 1723580643-467814 X-HE-Meta: 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 piIRXkiy uuR7aaTo1V7paTyKZTdxmT3z3ESPJvjyAsTyYfmnaU+EuAA9TEBOQI7sYkbFxaSD9m5G5YTMdvbzly8Gqd2SYcgz2IxxnnbDOWh0RletXBKqMYndT44/yPzGKqYj/wYNrNB0zvWbtPkQHGC9wrgdbAGjhW1Hf+oAbhrlea0dsxeBck+emn4ZXNZG+oYWN3hiNIt+sPhmmQX25OeNn+Dvkhqn8hIJpv1WOQTTaxs322ymIRix6lI8waGPFoCpQuFa7TncviGXLQuyCB2Ov0gUNtwMiQfMzZvPVLIAz5fBj1j69avKFClIN1uaL1YKFt1+Juo3IU23geo/thFabNZD+kTvtgeRhCOImi2NR8Z3HwbF6NCSfDqb419iS0UUle5c6OhLnM9JytCLehPWRN6gP0Q40Ftg/WC8oAIxv38bshXdSVvL5fC6LXQgamA== 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 16:01, Linus Torvalds wrote: > 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? ARCH_HAS_ACQUIRE_RELEASE is fine, but the help text for this Kconfig option should clarify this preference as both the ARCH_HAS_ACQUIRE_RELEASE and the !ARCH_HAS_ACQUIRE_RELEASE code are valid. Cheers, Longman