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 9AE32EB64DC for ; Tue, 11 Jul 2023 09:39:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229742AbjGKJjL (ORCPT ); Tue, 11 Jul 2023 05:39:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59096 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230258AbjGKJjJ (ORCPT ); Tue, 11 Jul 2023 05:39:09 -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 3003710F1 for ; Tue, 11 Jul 2023 02:38:38 -0700 (PDT) Received: by mail-ej1-x64a.google.com with SMTP id a640c23a62f3a-993c2d9e496so290775166b.0 for ; Tue, 11 Jul 2023 02:38:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1689068316; x=1691660316; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=MBgmP0PJ2iUxFozvmmmYwZcxD6aJZ5z3cAsE+Xf1as4=; b=ZW2z8hOF1WRo80QrafK0JkXI57zkP9C7rM4fPyaa1AxBIiYredVLSA46iO2WpKHuUc +geNJaQzzi4cADrIqii2oBP5uLE/koheH+H1dMZyW8g5P9DFudIKPnlIzlwxlbWP54xt x6BbXLcZAVMRHMd+S6GmIEOg64evUP91Wj0dubwkkXgIVikM48pkRssOCeOwdeiI6bvT Jd1XfA7QMJ5YTREszwAJD9SPurKXk6nAfSemFpFTVlt9+TveOlmzcZxNfHAc1HQzr5pc s+HBC/OZwUOw33LF1YhiwJbMowsX/6i0a4QjDkKySJUXnHVTvlV6uJVuyzYybavzJcs7 a3wg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1689068316; x=1691660316; 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=MBgmP0PJ2iUxFozvmmmYwZcxD6aJZ5z3cAsE+Xf1as4=; b=HrontAARPejwunxqfp7OPLlsr3TY72t5p7Mym//vQHqCXyYGBLs2ZXylQ3k0zkUfeO clEytZS7S1jOQKFN30Vp57Lll2bZ0ig07e00sYUUPZ48DZin6MsK422D4SKMlFG3vJ9H BQEyJSVJiQVul+iSZjt5lnszDir+b9J9oG2FF8s7HO/WYbsoFFAD7Qi30O2MqGBwszXD C182CN+OOeHrVh8NO3v270qgBrJuA1Ug5AsillV3Lw25B7HJROElgeUteooahOCYrPPw pQyQYL/Au3yRWvi1Afe0RfuuP861G9WSr9EUGnS46/n/kej3CGVFN5UBkasHW/a0UXUT VRPQ== X-Gm-Message-State: ABy/qLZalT28lSCYOMdTVw1to2UGW+4Zev4R0e4JgVVtVEhUy01w9yv4 G/ZOlTbLuRhNeqzFMwnj8eES255wtpmtcGawCnbQ9eEmtsnonpE6HuB5w5J3D8mNU9airsYRruj f5v0cUymC2IN5Cq0O4Tz3e2DacdaFNFcgmODYC2LugY60EksJ1Tn5f46b6HHICp9+IFrWNcDFAM qryvXH X-Google-Smtp-Source: APBJJlHRkdAxXDJOzU6GNe5HqX7AK0yIjF3AgFRk6eKq/3smS8Hc+PazHrra3a28syJBLWIV2m+lLAmY5hhiBQU= X-Received: from aliceryhl.c.googlers.com ([fda3:e722:ac3:cc00:31:98fb:c0a8:6c8]) (user=aliceryhl job=sendgmr) by 2002:a17:907:a047:b0:98e:4e80:f0e3 with SMTP id gz7-20020a170907a04700b0098e4e80f0e3mr50726ejc.9.1689068316182; Tue, 11 Jul 2023 02:38:36 -0700 (PDT) Date: Tue, 11 Jul 2023 09:32:56 +0000 In-Reply-To: <20230711093303.1433770-1-aliceryhl@google.com> Mime-Version: 1.0 References: <20230711093303.1433770-1-aliceryhl@google.com> X-Mailer: git-send-email 2.41.0.255.g8b1d071c50-goog Message-ID: <20230711093303.1433770-3-aliceryhl@google.com> Subject: [PATCH v3 2/9] rust: sync: add `Arc::{from_raw, into_raw}` From: Alice Ryhl To: rust-for-linux@vger.kernel.org, Tejun Heo , Miguel Ojeda Cc: Lai Jiangshan , Wedson Almeida Filho , Alex Gaynor , Boqun Feng , Gary Guo , "=?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?=" , Benno Lossin , Alice Ryhl , linux-kernel@vger.kernel.org, patches@lists.linux.dev, Wedson Almeida Filho Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: rust-for-linux@vger.kernel.org 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: Gary Guo --- v2 -> v3: * Use `transmute_copy` to convert metadata. * Use `add` instead of `wrapping_add`. * Dropped Reviewed-by from Martin and Benno due to changes. rust/kernel/lib.rs | 1 + rust/kernel/sync/arc.rs | 49 +++++++++++++++++++++++++++++++++++++++-- 2 files changed, 48 insertions(+), 2 deletions(-) diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index 788914fd935f..6963d11092c4 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -17,6 +17,7 @@ #![feature(const_refs_to_cell)] #![feature(dispatch_from_dyn)] #![feature(new_uninit)] +#![feature(ptr_metadata)] #![feature(receiver_trait)] #![feature(unsize)] diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs index a89843cacaad..4046b534e53b 100644 --- a/rust/kernel/sync/arc.rs +++ b/rust/kernel/sync/arc.rs @@ -24,13 +24,13 @@ }; use alloc::boxed::Box; use core::{ - alloc::AllocError, + alloc::{AllocError, Layout}, fmt, marker::{PhantomData, Unsize}, mem::{ManuallyDrop, MaybeUninit}, ops::{Deref, DerefMut}, pin::Pin, - ptr::NonNull, + ptr::{NonNull, Pointee}, }; use macros::pin_data; @@ -212,6 +212,51 @@ unsafe fn from_inner(inner: NonNull>) -> Self { } } + /// 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 { + let refcount_layout = Layout::new::(); + // SAFETY: The caller guarantees that the pointer is valid. + let val_layout = Layout::for_value(unsafe { &*ptr }); + // SAFETY: We're computing the layout of a real struct that existed when compiling this + // binary, so its layout is not so large that it can trigger arithmetic overflow. + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 }; + + let metadata: ::Metadata = core::ptr::metadata(ptr); + // SAFETY: The metadata of `T` and `ArcInner` is the same because `ArcInner` is a struct + // with `T` as its last field. + // + // This is documented at: + // . + let metadata: as Pointee>::Metadata = + unsafe { core::mem::transmute_copy(&metadata) }; + // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the + // pointer, since it originates from a previous call to `Arc::into_raw` and is still valid. + let ptr = unsafe { (ptr as *mut u8).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 -- 2.41.0.255.g8b1d071c50-goog