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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: <20260217091723.GU1395266@noisy.programming.kicks-ass.net> References: <20260213095557.GS2995752@noisy.programming.kicks-ass.net> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: B336816000D X-Rspamd-Server: rspam07 X-Stat-Signature: b3k3w65fbn1j5pnuytynowu53ndsjgks X-HE-Tag: 1771319848-372593 X-HE-Meta: U2FsdGVkX1+6TQKR2Gue2qKAKvK4vvr07fndYKHUhmaFbMHRhuJQaWT0Q5/JapVYgvFaYTkl99eJ8bFw0xKsKlNxRuRkWRAGmevU8NW2WknS9PZUd1YsDY8ykLP8R21OW16fnIwg8G7iWYAI+EHyv11oAJ8G6acFjUu+ddPPLmlcQIu8yq2hgaIIvhg//xJw0IimwOF+f8yAD0QL5KV0M5yqQu+aHqAVSs5ZC54i53cU4gVFDC9dNlJHDDaw8y8Xcb2UF/UHKz3QVQn8dQ/hO8y/O617+hMEGkSQGC4F1FNoSac0yq8UbaXKwsPMAFkFLrIE75YKWipLwJ7LdywrwhYHGYzOkJZX8jTsHzsNXlhX3/tJNkcgLmIxCgdwSBc7PrtEY/5baV5OmNnW2jNnMwRLrvKuRKd7vGdNMHiTfIM7oDEEfN+qdpz5HJStVdlcQ5hYY9Tc/3TR/9qouhZRC/8Ambtk+1+WDQVXkoYsuEV7qtK3Ox2JaCzozPb83zzv8sIDWXj1eHAghD55JMYF5VQmPPhyv8kf+3LtuT2t6YHWJ5ngt78jRS3LnCPo2JB5El7MzsoRphMufrWdTtKJ59Ur78i/anweeXAoHPXgrY5z9BoQV53wdTwv/eHaCEGuVj7/yFdVb9j6T9w2Gw+9z80HXlGd+qHluA22O6HnmPffORQ1nd8RYWXqhmwJYjX6Icu3R18UX1Nx56VdsSEQZP3M2aMwJuh5eQM1oiyGWz0/wbUGwCDWTe35D1a5K99hffDkvTluu99RirNV8m2qnmIlsOT9GzMIsYW6bTUZzf01PR2nEtJej//rTB/cFQEZubX5rt9JGlVitjD6YqF5o3hEipogineYB4w2gj99Q4yP3Fvpq9xoP3ZNt9RzH7GXbSR4AbPCtfKKqbzHerVjnDIt2LHPKjZx/nrtN7fbTXVXn2GijivvrLw8HgzLvZ67BR+Z/qd543bWd+1GT+G 5jN4GhE7 dFn5rhRcWbbrBInLnXHZVtQTT0aFgm5rBhXTDVJ4W7HtKDpS9y3WcapO8EPClYbsDhHLHtN88iNh65MmpHmqlRsxtr+bizO1vNVrFxAmAAjDDQndX24adR5EA+Tn/IBCQWpF/a0Pp6V3zFgPKwTyBe+HCucp4Z10ZHeMT7iKcxESlAqTYo40nRhItWY6SPbjsjX37yM7dp4eZiDRCA8uV8L61yBuTAmxnJNjmU1vGU1IyLe3XJ490xKSq1iIuirbuChMC87Cf2+BW9vBpocbseOrsxktvesl3ub/3M+JJDWjhULsrxzH0Jd+lI5iuzn4OIhCpgDQlG0V/kDqq3+/Grn9svA== 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, 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.