From nobody Thu Nov 28 22:37:52 2024 Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A873918890A; Sat, 28 Sep 2024 13:53:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=167.114.26.122 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727531635; cv=none; b=SeoksmkRSS8c9TS5RCU9/UBZ4XtO6ZTcYGqCsZ9yV6ie65fN500RRgo/PQEerbG6HxzOQuuHKTE31drT1Qsa0VOs39Z7dzZgc0dp8QvPuopCKFS7/4M/Qlv5+zNTjA3FVd6e259PyYe8Nh58UAMFO0PLF7c35UX7VH6who7Enww= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727531635; c=relaxed/simple; bh=adr64DJrpL036YYqX3VlBhGw9oL6SMunripgcF3S5mk=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=MndRobE69JjEOnZZ7nMrqmM6Fad09Uzqqm34pyUv7x14Febvw+AXaXLVAlusvYI/pp5bfXO46kU92Pui7IKf15PbG3toyTbOe3bGjQOZji08bQYprfQeZRnLXUdYYiOGWLKxrOSyeulg+lNEMOscFR53SXzR47WI+BO6PgsOoAo= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com; spf=pass smtp.mailfrom=efficios.com; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b=VZ2bjJPy; arc=none smtp.client-ip=167.114.26.122 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=efficios.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="VZ2bjJPy" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1727531632; bh=adr64DJrpL036YYqX3VlBhGw9oL6SMunripgcF3S5mk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=VZ2bjJPyzV8LOr/fUK3q5+I26yAxJvJ8sorbWZ4MKmbGYZTIJ/fIr7Hyx30CAKpxO WpIBkNQNlW+jolCn9hc284TT8OGzd/5YzG2DKaCz5NhwzMJGPWiHb3T3Q2ElKSsVQk iY5gtCZDD8laZdfbhHUFc1K4XKjgTuoGqs8JP62v3fuje2aC3KS1EKrX9G6HQmkKQ9 afMfgD+0hWceM6vNAUQGUEgDUjAWBhXzNeWOA2qANQmQZadycqwMlcCHN1lkspEiZU i4ggfozc1u0PYWQlkV8coab3oTkaE8OMtg/HAVLx7XKlwkGOVU3RwUet1Tk7XSLeES KNMf+4kwaXmuQ== Received: from thinkos.internal.efficios.com (unknown [IPv6:2606:6d00:100:4000:cacb:9855:de1f:ded2]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XG8282zDgz1bsD; Sat, 28 Sep 2024 09:53:52 -0400 (EDT) From: Mathieu Desnoyers To: Linus Torvalds Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Greg Kroah-Hartman , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , Peter Zijlstra , Boqun Feng , Alan Stern , John Stultz , Neeraj Upadhyay , Frederic Weisbecker , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Gary Guo , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev Subject: [PATCH 2/2] Documentation: RCU: Refer to ptr_eq() Date: Sat, 28 Sep 2024 09:51:28 -0400 Message-Id: <20240928135128.991110-3-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20240928135128.991110-1-mathieu.desnoyers@efficios.com> References: <20240928135128.991110-1-mathieu.desnoyers@efficios.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Refer to ptr_eq() in the rcu_dereference() documentation. ptr_eq() is a mechanism that preserves address dependencies when comparing pointers, and should be favored when comparing a pointer obtained from rcu_dereference() against another pointer. Signed-off-by: Mathieu Desnoyers Cc: Greg Kroah-Hartman Cc: Sebastian Andrzej Siewior Cc: "Paul E. McKenney" Cc: Will Deacon Cc: Peter Zijlstra Cc: Boqun Feng Cc: Alan Stern Cc: John Stultz Cc: Neeraj Upadhyay Cc: Linus Torvalds Cc: Boqun Feng Cc: Frederic Weisbecker Cc: Joel Fernandes Cc: Josh Triplett Cc: Uladzislau Rezki Cc: Steven Rostedt Cc: Lai Jiangshan Cc: Zqiang Cc: Ingo Molnar Cc: Waiman Long Cc: Mark Rutland Cc: Thomas Gleixner Cc: Vlastimil Babka Cc: maged.michael@gmail.com Cc: Mateusz Guzik Cc: Gary Guo Cc: Jonas Oberhauser Cc: rcu@vger.kernel.org Cc: linux-mm@kvack.org Cc: lkmm@lists.linux.dev --- Documentation/RCU/rcu_dereference.rst | 34 +++++++++++++++++++++++---- 1 file changed, 29 insertions(+), 5 deletions(-) diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_= dereference.rst index 2524dcdadde2..c36b8d1721f6 100644 --- a/Documentation/RCU/rcu_dereference.rst +++ b/Documentation/RCU/rcu_dereference.rst @@ -104,11 +104,13 @@ readers working properly: after such branches, but can speculate loads, which can again result in misordering bugs. =20 -- Be very careful about comparing pointers obtained from - rcu_dereference() against non-NULL values. As Linus Torvalds - explained, if the two pointers are equal, the compiler could - substitute the pointer you are comparing against for the pointer - obtained from rcu_dereference(). For example:: +- Use relational operators which preserve address dependencies + (such as "ptr_eq()") to compare pointers obtained from + rcu_dereference() against non-NULL values or against pointers + obtained from prior loads. As Linus Torvalds explained, if the + two pointers are equal, the compiler could substitute the + pointer you are comparing against for the pointer obtained from + rcu_dereference(). For example:: =20 p =3D rcu_dereference(gp); if (p =3D=3D &default_struct) @@ -125,6 +127,23 @@ readers working properly: On ARM and Power hardware, the load from "default_struct.a" can now be speculated, such that it might happen before the rcu_dereference(). This could result in bugs due to misordering. + Performing the comparison with "ptr_eq()" ensures the compiler + does not perform such transformation. + + If the comparison is against a pointer obtained from prior + loads, the compiler is allowed to use either register for the + following accesses, which loses the address dependency and + allows weakly-ordered architectures such as ARM and PowerPC + to speculate the address-dependent load before rcu_dereference(). + For example:: + + p1 =3D READ_ONCE(gp); + p2 =3D rcu_dereference(gp); + if (p1 =3D=3D p2) + do_default(p2->a); + + Performing the comparison with "ptr_eq()" ensures the compiler + preserves the address dependencies. =20 However, comparisons are OK in the following cases: =20 @@ -204,6 +223,11 @@ readers working properly: comparison will provide exactly the information that the compiler needs to deduce the value of the pointer. =20 + When in doubt, use relational operators that preserve address + dependencies (such as "ptr_eq()") to compare pointers obtained + from rcu_dereference() against non-NULL values or against + pointers obtained from prior loads. + - Disable any value-speculation optimizations that your compiler might provide, especially if you are making use of feedback-based optimizations that take data collected from prior runs. Such --=20 2.39.2