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[209.85.208.173]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-538a0457c0esm424245e87.298.2024.09.27.13.24.24 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 27 Sep 2024 13:24:24 -0700 (PDT) Received: by mail-lj1-f173.google.com with SMTP id 38308e7fff4ca-2f761461150so38061461fa.0 for ; Fri, 27 Sep 2024 13:24:24 -0700 (PDT) X-Forwarded-Encrypted: i=1; AJvYcCVUQchrj2csCgN8KrTAi6F1D71N2DK4r1eMQIi44MMxzg/QeAmz0qMHldi8s1aematxR5LQueW7TQ==@kvack.org X-Received: by 2002:a05:6512:3095:b0:536:a5ee:bb75 with SMTP id 2adb3069b0e04-5389fc3f106mr5002054e87.21.1727468663689; Fri, 27 Sep 2024 13:24:23 -0700 (PDT) MIME-Version: 1.0 References: <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> <0b262fe5-2fc5-478d-bf66-f208723238d5@efficios.com> <7e1c8a5e-c110-414c-8fb2-022eacc2bd4a@efficios.com> <34ec590c-b109-44a0-8bfe-8aafc6e7ad64@efficios.com> <1b04e35c-8994-48d9-907a-966bb4dfabaf@huaweicloud.com> In-Reply-To: From: Linus Torvalds Date: Fri, 27 Sep 2024 13:24:07 -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 (Sony)" , 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 , Greg Kroah-Hartman Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: 89D781A0006 X-Stat-Signature: t8n3azg88y95ptrg5u87s4rkmsa4ko7y X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1727468667-723183 X-HE-Meta: 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 T5T8pk9B 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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 Fri, 27 Sept 2024 at 12:28, Linus Torvalds wrote: > > Dammit, people, read the code I posted. Actually, no, apologies. You were right, and I was wrong. It does need both of the sources of the comparison to be hidden, because while even hiding just one side makes the comparison result "meaningless" as far as the compiler is concerned, the compiler will still have generated a pseudo for the hidden value, and might decide that it can re-use that pseudo for the non-hidden pointer if the two then match. So yeah, the example function I posted should be safe, but my "you can probably make do with hiding just one side" was actually a bogus and invalid optimization. You do need to hide both sides. Not just for clarity, but for correctness. Linus