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[209.85.218.44]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a93c2978beesm17487066b.147.2024.09.26.09.54.50 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Sep 2024 09:54:51 -0700 (PDT) Received: by mail-ej1-f44.google.com with SMTP id a640c23a62f3a-a8d24f98215so166575766b.1 for ; Thu, 26 Sep 2024 09:54:50 -0700 (PDT) X-Forwarded-Encrypted: i=1; AJvYcCUQpbSHsx8CwY3anz6/3mRFhwSx14b2X5yq7LraSivZvfwPSQmSrY4EXLNLoWzny5MXT5LrjtFUDQ==@kvack.org X-Received: by 2002:a17:907:7e8e:b0:a86:9d3d:edef with SMTP id a640c23a62f3a-a93c48e8c12mr13943566b.12.1727369690656; Thu, 26 Sep 2024 09:54:50 -0700 (PDT) MIME-Version: 1.0 References: <20240917143402.930114-1-boqun.feng@gmail.com> <20240917143402.930114-2-boqun.feng@gmail.com> <55975a55-302f-4c45-bfcc-192a8a1242e9@huaweicloud.com> <4167e6f5-4ff9-4aaa-915e-c1e692ac785a@efficios.com> <48992c9f-6c61-4716-977c-66e946adb399@efficios.com> <2b2aea37-06fe-40cb-8458-9408406ebda6@efficios.com> <55633835-242c-4d7f-875b-24b16f17939c@huaweicloud.com> <54487a36-f74c-46c3-aed7-fc86eaaa9ca2@huaweicloud.com> In-Reply-To: <54487a36-f74c-46c3-aed7-fc86eaaa9ca2@huaweicloud.com> From: Linus Torvalds Date: Thu, 26 Sep 2024 09:54:33 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers To: Jonas Oberhauser Cc: Mathieu Desnoyers , Boqun Feng , linux-kernel@vger.kernel.org, rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev, "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Peter Zijlstra , Ingo Molnar , Will Deacon , Waiman Long , Mark Rutland , Thomas Gleixner , Kent Overstreet , Vlastimil Babka , maged.michael@gmail.com, Neeraj Upadhyay Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 76FC6180007 X-Stat-Signature: 6yp15dernw5mho393z145ymn4qmiggmj X-Rspam-User: X-HE-Tag: 1727369693-497126 X-HE-Meta: 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 m+tPd/Ce 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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 Thu, 26 Sept 2024 at 09:40, Jonas Oberhauser wrote: > > Boqun seems to be unhappy with a barrier though, because it would > theoretically also forbid unrelated optimizations. Well, doing a "barrier()" is kind of a big hammer thing, but honestly, I don't think we've ever seen any real situation where it makes a noticeable difference. Yes, it can pessimize compiler output more than strictly necessary, but the kind of code generation issues it causes tends to be the non-problematic kind (and particularly the kind that even a trivial OoO core will deal with well). We do have some more directed compiler barriers available, and this code might be able to use OPTIMIZER_HIDE_VAR() for example. It's kind of a "single variable value barrier". Honestly, we don't use it much. It just tends to be _too_specific. But it is there if somebody wants to use it. > But I have not seen any evidence that there are any unrelated > optimizations going on in the first place that would be forbidden by this. Compared to something like "smp_mb()", which is not just a compiler barrier but also generates typically very expensive instructions that completely mess with an OoO core, a regular compiler barrier is a complete non-issue. When you have those two close to each other, you'd have to make up some very odd situation where the plain "barrier()" is even noticeable. Linus