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Howlett" , Miguel Ojeda , Boqun Feng , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Trevor Gross , Danilo Krummrich , Will Deacon , Mark Rutland , linux-mm@kvack.org, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] rust: page: add byte-wise atomic memory copy methods Message-ID: <20260217093344.GO1395416@noisy.programming.kicks-ass.net> References: <40xUh92AU5E9oFxQrdej-AXVg76jmaWGKXZMLoOHXe35Lw9x_eNEoLup9bB60LyGZ_0USPmoxr-9hE3ujA67cQ==@protonmail.internalid> <2026021343-germicide-baritone-efe8@gregkh> <877bsgu7fb.fsf@kernel.org> <2026021313-embody-deprive-9da5@gregkh> <873434u3yq.fsf@kernel.org> <20260213142608.GV2995752@noisy.programming.kicks-ass.net> <2026021311-shorten-veal-532c@gregkh> <20260217091723.GU1395266@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260217091723.GU1395266@noisy.programming.kicks-ass.net> X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: EA45D180011 X-Stat-Signature: xam6puy969st4jx6xgygtrok6ghfyhow X-HE-Tag: 1771320834-981697 X-HE-Meta: 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 S/6MwBtc oNoonlRsNYiiV/jGIXTDD8EHX3TE7u50iT3qzfhp1aQ8iC4D1GoZ7rLA7Xy6mDVgBB9haZ6fEgZMvkrLJ4r+kV5eIWaOthX5d15VUUQjzeQpw3aOwqEawfqR7//ppzx6r1f3BLsWKMI2sYRpMYX4vPqTWs+Y8+GlOw1m/QYFZq3vpQwYquG2CZayJOL45x+Pqpx6j4rpMW8kjpvBRKSwyDAF+DHPC0RCwWbN4HhL1ip/+5j+vx8HzaPzOdvPQuMWImVOuZDtwpquWMonaJOfI9vcQMqm+kpMnwVUjJSNqAWK9SiRHv+oIeoQuJHNFlPFQLE1Q7nxC0rVdM33052k/YJ1p/A== 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 Tue, Feb 17, 2026 at 10:17:23AM +0100, Peter Zijlstra wrote: > On Fri, Feb 13, 2026 at 07:45:19AM -0800, Boqun Feng wrote: > > > > > Suppose the memory was 'AAAA' and while you're reading it, it is written > > > > to be 'BBBB'. The resulting copy can be any combination of > > > > '[AB][AB][AB][AB]'. Not one of them is better than the other. > > > > > > > > The idea is if using Rust's own `core::ptr::copy()` or > > `core::ptr::copy_nonoverlapping()`, you may get `CCCC`, because they are > > not semantically guaranteed atomic per byte (i.e. tearing can happen at > > bit level, because they are not designed for using in case of data > > races, and there is no defined asm implementation of them, compilers can > > do anything). > > How the heck would they do out-of-thin-air? Any memcpy() implementation > that can cause that is insane and broken. > > Compilers are broken crap if they do this. The only way I know for a compiler to cause out-of-thin-air is speculative stores -- those MUST NOT happen. Just like hardware must not do them. We must all collectively say NO to a compiler that does these. Life is too short to try and write concurrent programs with a compiler that does this.