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Thu, 02 Jul 2020 04:39:26 -0700 (PDT) MIME-Version: 1.0 References: <20200630173734.14057-1-will@kernel.org> <20200630173734.14057-5-will@kernel.org> <20200702093239.GA15391@C02TD0UTHF1T.local> <20200702094833.GA16248@willie-the-truck> <20200702111832.GC16418@willie-the-truck> In-Reply-To: <20200702111832.GC16418@willie-the-truck> From: Arnd Bergmann Date: Thu, 2 Jul 2020 13:39:10 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation To: Will Deacon X-Provags-ID: V03:K1:vXzhAAyY33wx2UhxNYgD+O54uPnvYCDoG+821aqMtCk/V3lvoQm SEAaa27qoREzC8J/TXitF7b6SFRcgO+EUoez5C0kyX4PFvEaJerox/mpbWnyxLKYsPeRNb8 ASRCJ8LR6hFtGmXqAOSaBNWLUNHieaXfvmRolVtwS7Be//Cebjztm2Buj2MktJg85gieubg Oal5L6J/OxwLAx99iW8ig== X-UI-Out-Filterresults: notjunk:1;V03:K0:T2Zw8GEJ7ko=:hd+LQOI56n9oTo6irSSOmr 5taeDgCTwlawkApTIxaR4VVX7w7nwFglvs40E+yFoq1hNSHvrJNCeoDFUMm+Snia0PY8z8mgQ /OAZEyxIDdSF2E8xMwbBVeOq3NKVyF5CyCbMjFZh4LvzEC2/B6cSmmbu/hGXoE7eFY2azPJxF GfMfAgQgIDyeYecMYPRAmx5SbvJsdF5s3FlnKLpMWmftL4OkwoK7cAm677JF6Fzg5WEhzFXMP NSc5T+VcJ4G+ovd55x3PNh4zdcqqMQrREAGHAcJOO8c5qVYAsxw4Ct1zPsfE2YpQFNjE28iq4 bTIWXk8rA7mPGrpqc2179PktSY/M9dCJvpCnAQmEiBpFOmly3uDdZFGSI7q3COClm+pmty0yI asebsmh/zBZlXeSpW9W0QY5ZNUqcV7ElZQROw1K7KvP2c0R42lQfu5lqDicLfamDVgkuDoMzj 8fVF3GRF1rvBsGleAbgq5wP1EeEKczCVYo11oXo+1oeS6jhNcJqblrMBUsatAe9InYlWOGtw+ QqB/2T4uHEVSYbmD6hZxqDtsshcz5+xLMV3oz2y8COKheLiB9o9M8ZINigeeowqEoKl6EwpS6 pfo16tl5xq2RtHWGnXGkhJsasMqZDnx7huvAvl7OSGAs/RrGR4HLLJoY/SaABeO6S9Bw+rCxD LlOCTUXg7upQ8zYWd6+iLKp9aJsa5ind++PMTyJodG1vpdHNuwH9ZBqYfzWHBiMqtDAFA0PhE n+B7/8zM/SawbAYF3Rh1eN328rlN0hsk4aHOxXGK4gAp/6BAIF8TfFaqRZZ/pwI0BkXh4UVXl OAlNnyzh2AXSW6zI1vAAS+h0RXFzFOCg3yyVhp1ohWQmW9Kj5c= X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200702_073932_998899_342C6CB4 X-CRM114-Status: GOOD ( 15.76 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Marco Elver , Kees Cook , "Paul E. McKenney" , "Michael S. Tsirkin" , Peter Zijlstra , Catalin Marinas , Jason Wang , Nick Desaulniers , "linux-kernel@vger.kernel.org" , Josh Triplett , Ivan Kokshaysky , Sami Tolvanen , alpha , Alan Stern , Matt Turner , virtualization@lists.linux-foundation.org, Android Kernel Team , Boqun Feng , Linux ARM , Richard Henderson Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Jul 2, 2020 at 1:18 PM Will Deacon wrote: > On Thu, Jul 02, 2020 at 12:08:41PM +0200, Arnd Bergmann wrote: > > On Thu, Jul 2, 2020 at 11:48 AM Will Deacon wrote: > > > On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote: > Not sure I follow you here, but I can confirm that what you're worried > about doesn't happen for the usual case of a pointer-to-volatile scalar. > > For example, ignoring dependency ordering: > > unsigned long foo(volatile unsigned long *p) > { > return smp_load_acquire(p) + 1; > } > > Ends up looking like: > > unsigned long ___p1 = *(const volatile unsigned long *)p; > smp_mb(); > (volatile unsigned long)___p1; > > My understanding is that casting a non-pointer type to volatile doesn't > do anything, so we're good. Right, I mixed up the correct (typeof(*p))___p; with the incorrect *typeof(p)&___p; which would dereference a volatile pointer and cause the problem. The code is all fine then. Arnd _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel