From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 92282C77B7A for ; Wed, 24 May 2023 11:11:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234651AbjEXLLf (ORCPT ); Wed, 24 May 2023 07:11:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42500 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234552AbjEXLLd (ORCPT ); Wed, 24 May 2023 07:11:33 -0400 Received: from mail-ed1-x549.google.com (mail-ed1-x549.google.com [IPv6:2a00:1450:4864:20::549]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7B0FAC5 for ; Wed, 24 May 2023 04:11:31 -0700 (PDT) Received: by mail-ed1-x549.google.com with SMTP id 4fb4d7f45d1cf-50de84a3861so865903a12.3 for ; Wed, 24 May 2023 04:11:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1684926690; x=1687518690; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=4mh/uplWXEJG8R+eeG89iIATTJ5tXDpqPwH0/Cpz+94=; b=Ealvqf6m5eTtrTJw51t/UJcXmkP4y9/lCnZH/rfLC2p1LCmw5L2podZOi9x4uyfp42 8BpGUAGcvSr0zooJbm/yd+HMtz9kh6YAJBttV5LW1foF3EEDwcfuUH3MDfcTqjtTWwrL cwEdXAixIuP1wKTZnHMAobXmu+IjqV1ZJyo+JgCgctPEtbFTvfDWnxKN2tqKhvw8tDrn o1RYy1vGzVcxOpH8pwVnpRjAo5boR7L7fvzosXftH8G+3aRDeftjnvZ7h4vW0xNCfRMd M+VSr+3qH7pw3z17P0nY/78Vbb3ODNQ8MsOH6O3UnwVtRV83T4Wl4/+urGCkxQzRwGrs sWPA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1684926690; x=1687518690; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=4mh/uplWXEJG8R+eeG89iIATTJ5tXDpqPwH0/Cpz+94=; b=efLm9wm4G6XBj7cxgwwiCeSJyoOwJIGd3Ql9Vqw1zaO3kU4CLox1nC2hRcUt0FuVZ4 PV8dNFMGyxCsN94U8E2Fg/VQJoY9Xfjh5XXv1/42Yyymug4s9lXGQ2Ijp8cEL5WTVUYk dLKuGy8APSOY2b6CbqrtqTa3FjzeXBGN8SXfT0ccnrvVQNgRblWfxR5UUqdT04iGU2ZC 23Vsa6ugQL9f9gLDGit3JtKo6NgXrf/FEKHUeC+3qC4/79XaBt0TuAKxR9QeoBPBKmi5 lwLmRM4aAn0lk/zGXVdAlAQtVRok7RodxR/Ckv5DEc72lSE91XODqQIis7bBJ0jWi4si eBnw== X-Gm-Message-State: AC+VfDzY6scPlnmhsTIeE7ffiRq0sPm576MNQPpmyh+qQzSpTeVjjeZd 7Br508IwBtdJlzRlGd2I/8SpTGX67Tbi5no= X-Google-Smtp-Source: ACHHUZ5+4OdHMonxT5AAGoSedUaFjM79+wW2g2Tg0tZikEMfewz+SKNu6+lWkRHbxOW7yIJB/AFDxLBZhC6BAz0= X-Received: from aliceryhl.c.googlers.com ([fda3:e722:ac3:cc00:31:98fb:c0a8:6c8]) (user=aliceryhl job=sendgmr) by 2002:a50:c34a:0:b0:504:89cf:8170 with SMTP id q10-20020a50c34a000000b0050489cf8170mr758066edb.4.1684926689900; Wed, 24 May 2023 04:11:29 -0700 (PDT) Date: Wed, 24 May 2023 11:11:27 +0000 In-Reply-To: <87edn6do1b.fsf@metaspace.dk> Mime-Version: 1.0 References: <87edn6do1b.fsf@metaspace.dk> X-Mailer: git-send-email 2.40.1.698.g37aff9b760-goog Message-ID: <20230524111127.2491643-1-aliceryhl@google.com> Subject: Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}` From: Alice Ryhl To: nmi@metaspace.dk Cc: alex.gaynor@gmail.com, aliceryhl@google.com, benno.lossin@proton.me, bjorn3_gh@protonmail.com, boqun.feng@gmail.com, gary@garyguo.net, jiangshanlai@gmail.com, linux-kernel@vger.kernel.org, ojeda@kernel.org, patches@lists.linux.dev, rust-for-linux@vger.kernel.org, tj@kernel.org, wedsonaf@gmail.com Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andreas Hindborg writes: > Alice Ryhl writes: >> + // This preserves the metadata in the pointer, if any. >> + let metadata = core::ptr::metadata(ptr as *const ArcInner); > > I can't follow this. How does this work? `ptr` was for field > `inner.data: T`, but we are casting to `ArcInner`. > >> + let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut (); >> + let ptr = core::ptr::from_raw_parts_mut(ptr, metadata); > > Metadata was obtained from a pointer pointing to `inner.data`, we then > move it back to beginning of `ArcInner` and then reconstruct the > potentially fat pointer with metadata from the pointer to `T`? How can > this be right? The metadata of a struct is always the metadata of its last field, so both `*mut T` and `*mut ArcInner` have the same metadata. Because of that, moving the metadata over from one type to the other is ok. The reason that I cast to an `ArcInner` pointer before calling `metadata` is because I get a type mismatch otherwise for the metadata, since the compiler doesn't unify the metadata types when the type is generic. Alice