From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-lf1-f42.google.com (mail-lf1-f42.google.com [209.85.167.42]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C31BABE50 for ; Thu, 25 May 2023 07:49:32 +0000 (UTC) Received: by mail-lf1-f42.google.com with SMTP id 2adb3069b0e04-4f4b2bc1565so2021601e87.2 for ; Thu, 25 May 2023 00:49:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=metaspace-dk.20221208.gappssmtp.com; s=20221208; t=1685000970; x=1687592970; h=mime-version:message-id:in-reply-to:date:subject:cc:to:from :user-agent:references:from:to:cc:subject:date:message-id:reply-to; bh=V1XvSVODDEA3TsNr4/6rD8fONkRZXOKHxfaQzrvBuWE=; b=Rb//MjVriA87cNvKfaQfy3k9aTXfLTkqIAAMtLnZJCSLwvy103u/pNn02HRykZaUUX 1vsVPWhDqU+OqtlMaTmprHYMgDKPpfCWb49z0qLWfQH45DLgWjAORZw3oExhyKG1IHSI J4fXuhJ8dMvB8XOb1vj/I6nA1/TKUznn+jJgljKwgqIUQLmIgBOCFrrgRiCVQwJ4digD b7vxocdF8bhn3v9RlkTFXzfH0YUs2UwotoGSt/gWnNoblBQfExdUtRP6J2BqCTLKf9+Z B5zPjKK+Mpt7ur40CI3YTFTWHXU9rIDR9C+hGWSfoUJKIjtUIV9NT7yndiav3lObKCV5 xEtQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1685000970; x=1687592970; h=mime-version:message-id:in-reply-to:date:subject:cc:to:from :user-agent:references:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=V1XvSVODDEA3TsNr4/6rD8fONkRZXOKHxfaQzrvBuWE=; b=PotJ8nocA7ndCHkMdVl9yLTi2if7dfutbdgXY+OHZx/UYbvkNmRNtIuYIrHyHBE9lj G7Qs0JV1vQ2AQOmIq8D963IrTKaAoIZHyHBnJiR2PGQYyNR1Ly+/O7hS3veBcU2ABcmj 3zlKi0IUn8C6mw+pzgh2TQw0rObne1mQ3Ivk1D6TNebhmGDiDy5zrbcJCPOclHkTcsnJ v0m+pO/8GHNZMh7ZcMel4f1uNK5Yn/fLT8MIkZARFpb2l6jq9cEvjjPm7sNBWViwvuP8 MOeA1c4VkB0S60wK0SocuVuhpnQZ8RNKyeMehwH5qj1YkZi8sfuvWGdtQ6kslWhPl/Xa oUAw== X-Gm-Message-State: AC+VfDy/XZyxti1bLFEkW8QMDDHBv3caVKMwS/fupTkUspt1XwMkLTtM eWfE5FAv8Pzlxke6hHFPNwb6qw== X-Google-Smtp-Source: ACHHUZ4kdZAHbddsr82/JU+BYvNmlUQoxgJp+jPn0fQ2cDA6MucbPgKmHAJbM5tZHMio1jw9kM0xoQ== X-Received: by 2002:a05:6512:21a6:b0:4ef:f3bf:93a6 with SMTP id c6-20020a05651221a600b004eff3bf93a6mr6505557lft.51.1685000970268; Thu, 25 May 2023 00:49:30 -0700 (PDT) Received: from localhost (77.241.128.147.mobile.3.dk. [77.241.128.147]) by smtp.gmail.com with ESMTPSA id t17-20020ac25491000000b004f3b6e71eb1sm108281lfk.291.2023.05.25.00.49.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 25 May 2023 00:49:29 -0700 (PDT) References: <87edn6do1b.fsf@metaspace.dk> <20230524111127.2491643-1-aliceryhl@google.com> User-agent: mu4e 1.10.3; emacs 28.2.50 From: Andreas Hindborg To: Alice Ryhl Cc: alex.gaynor@gmail.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 Subject: Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}` Date: Thu, 25 May 2023 09:45:29 +0200 In-reply-to: <20230524111127.2491643-1-aliceryhl@google.com> Message-ID: <87a5xsetq5.fsf@metaspace.dk> Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Alice Ryhl writes: > 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. OK, cool. In that case, since this is common knowledge (is it?), could you maybe include a link to the relevant documentation, or a comment indicating why this is OK? BR Andreas