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[2620:137:e000::1:20]) by mx.google.com with ESMTP id m12-20020a633f0c000000b00528a60ab393si21662381pga.314.2023.05.17.13.37.14; Wed, 17 May 2023 13:37:55 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20221208 header.b="BnGU93/l"; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229785AbjEQUbl (ORCPT + 99 others); Wed, 17 May 2023 16:31:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56204 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229514AbjEQUbc (ORCPT ); Wed, 17 May 2023 16:31:32 -0400 Received: from mail-ej1-x64a.google.com (mail-ej1-x64a.google.com [IPv6:2a00:1450:4864:20::64a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4203F30DE for ; Wed, 17 May 2023 13:31:30 -0700 (PDT) Received: by mail-ej1-x64a.google.com with SMTP id a640c23a62f3a-969f24626fbso68332766b.0 for ; Wed, 17 May 2023 13:31:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1684355489; x=1686947489; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=1m2NIZThmeCBayz0liS/tGETC/9gj7ELeSxPKaC6Yh8=; b=BnGU93/l6zgPX74n1BWhPYto1+DmrAaRJmje1ktjtxvvMVe5/ufxtkwfhHuGWoWdhk sTD0ze0cB+pBIMUh0eC20QGkHMtXngl1qheah8sXtY3xJUm4p2C/jy+3agGa/nXWMaBI cjXQYFgEMFGWjq0AK5WF2hb3rxsDq2qWXzEhoziYVq2kaalDDx9UX4nj4NQFwlvAumU+ X8d+B3yeLFzGBO3Z8qQeVsV6ID5+4q96aaX8eg1fnDPAsOkB3kNDdNSo2o+kmZH4J8aw 91bpLwB4E7EfRot8PStAuoblk4n4X2GSjicq/7i8QVuKgUhHDcQKGNJycsRIbr49laKs oJ7g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1684355489; x=1686947489; 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=1m2NIZThmeCBayz0liS/tGETC/9gj7ELeSxPKaC6Yh8=; b=Zl71juEiWgWapMieElwxyxvVlTgo7rZXGmSnDEl5iVUZUtXdGaJ0J8wium9eKnvjJG l8WIx8E+Y3+NYHhEDMO/JVB66nrlztT+q48BR5WxYF/2jh0RQPfYVSZBdZmLSWZEr5Nc pww5aVjUiac5OGx9EIs5vG0djJ6WUaIzm95Kku7G5cLi0mZ3Tc7kWQpD3n1G7eoO3RcE Yq85DoQVOkz7yNTdIxOtrByHjwC19OxQH3R24ApRTq4wHMfPbSLtNfHBJndSpW9HH1JG 7JwVs8neLGmiMSNeO38kZX3p/DbCGdZTg5uRF7VmjyIp0WNgg1OCeovMfmWeu6/KArk3 TYhA== X-Gm-Message-State: AC+VfDwxCnQWukm2fhCOSsmlU2PCqGADf9kO6U3gMSsIpj9DlMBSx9Pp /RyHxkWmBGaeM5NCnG9a8zaXCFsgoOFsjTU= X-Received: from aliceryhl.c.googlers.com ([fda3:e722:ac3:cc00:31:98fb:c0a8:6c8]) (user=aliceryhl job=sendgmr) by 2002:a17:907:2bf5:b0:94e:dcd5:2429 with SMTP id gv53-20020a1709072bf500b0094edcd52429mr1413196ejc.3.1684355488912; Wed, 17 May 2023 13:31:28 -0700 (PDT) Date: Wed, 17 May 2023 20:31:15 +0000 In-Reply-To: <20230517203119.3160435-1-aliceryhl@google.com> Mime-Version: 1.0 References: <20230517203119.3160435-1-aliceryhl@google.com> X-Mailer: git-send-email 2.40.1.606.ga4b1b128d6-goog Message-ID: <20230517203119.3160435-4-aliceryhl@google.com> Subject: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}` From: Alice Ryhl To: rust-for-linux@vger.kernel.org Cc: Miguel Ojeda , Wedson Almeida Filho , Tejun Heo , Lai Jiangshan , Alex Gaynor , Boqun Feng , Gary Guo , " =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= " , Benno Lossin , Alice Ryhl , linux-kernel@vger.kernel.org, patches@lists.linux.dev X-Spam-Status: No, score=-9.6 required=5.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, SPF_HELO_NONE,SPF_PASS,T_SCC_BODY_TEXT_LINE,URIBL_BLOCKED, USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1766175146251007014?= X-GMAIL-MSGID: =?utf-8?q?1766175146251007014?= From: Wedson Almeida Filho These methods can be used to turn an `Arc` into a raw pointer and back, in a way that preserves the metadata for fat pointers. This is done using the unstable ptr_metadata feature [1]. However, it could also be done using the unstable pointer_byte_offsets feature [2], which is likely to have a shorter path to stabilization than ptr_metadata. Link: https://github.com/rust-lang/rust/issues/81513 [1] Link: https://github.com/rust-lang/rust/issues/96283 [2] Signed-off-by: Wedson Almeida Filho Co-developed-by: Alice Ryhl Signed-off-by: Alice Ryhl Reviewed-by: Martin Rodriguez Reboredo --- rust/kernel/lib.rs | 1 + rust/kernel/sync/arc.rs | 44 +++++++++++++++++++++++++++++++++++++++++ 2 files changed, 45 insertions(+) diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index cdf9fe999328..82854c86e65d 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -22,6 +22,7 @@ #![feature(generic_associated_types)] #![feature(new_uninit)] #![feature(pin_macro)] +#![feature(ptr_metadata)] #![feature(receiver_trait)] #![feature(unsize)] diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs index e6d206242465..7c55a9178dfb 100644 --- a/rust/kernel/sync/arc.rs +++ b/rust/kernel/sync/arc.rs @@ -210,6 +210,50 @@ impl Arc { } } + /// Convert the [`Arc`] into a raw pointer. + /// + /// The raw pointer has ownership of the refcount that this Arc object owned. + pub fn into_raw(self) -> *const T { + let ptr = self.ptr.as_ptr(); + core::mem::forget(self); + // SAFETY: The pointer is valid. + unsafe { core::ptr::addr_of!((*ptr).data) } + } + + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`]. + /// + /// This code relies on the `repr(C)` layout of structs as described in + /// . + /// + /// # Safety + /// + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it + /// can only be called once for each previous call to [`Arc::into_raw`]. + pub unsafe fn from_raw(ptr: *const T) -> Self { + // SAFETY: The safety requirement ensures that the pointer is valid. + let val_align = core::mem::align_of_val(unsafe { &*ptr }); + let refcount_size = core::mem::size_of::>(); + + // Use the `repr(C)` algorithm to compute the offset of `data` in `ArcInner`. + // + // Pseudo-code for the `#[repr(C)]` algorithm can be found here: + // + let mut val_offset = refcount_size; + let val_misalign = val_offset % val_align; + if val_misalign > 0 { + val_offset += val_align - val_misalign; + } + + // This preserves the metadata in the pointer, if any. + let metadata = core::ptr::metadata(ptr as *const ArcInner); + let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut (); + let ptr = core::ptr::from_raw_parts_mut(ptr, metadata); + + // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the + // reference count held then will be owned by the new `Arc` object. + unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) } + } + /// Returns an [`ArcBorrow`] from the given [`Arc`]. /// /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method