From nobody Mon Feb 9 11:05:04 2026 Received: from smtpout.efficios.com (smtpout.efficios.com [158.69.130.18]) (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 3DDF522DFA4; Thu, 18 Dec 2025 01:45:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=158.69.130.18 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766022345; cv=none; b=XqoSEdN94BLEzrJcNIzlbagyxHE77QJTCO3fgxcGvqTlvrId+4LZnlLJhfSUbJlE9QCJ37lA1QzZy5QeTD+pfdWQu5RWjHdmlXS9MSlcLSGrOgMwhkRfoO6kktMjcQeZ1zHRclwSdsjxsj6ZO0ZwIkg07FPaz4JhECQnttEZbKc= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766022345; c=relaxed/simple; bh=+E3EpLSzGU9uk+qvSDSKfjoPetCv/kt4eujvMW/E3vA=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=s8Q1JZjWLPmegWixLp8dMrDO8iB3DWaHycJbJ9j/UV6dq4Zu4dYjE/YWXjn+fevr3thZZoxrDuOR5e7jV6d+ZXpdEYFrRbHJMCNd/YyfIwRkCHve/+fkxu8c56d2HCBFU+gBvowxowJUIYrldAWOTpexO+wnpncOWSx1Y9CV0aQ= 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=fpefWz2O; arc=none smtp.client-ip=158.69.130.18 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="fpefWz2O" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=smtpout1; t=1766022335; bh=eSepe1r3J5AkA/ggHcrTZYYdvp9Ci1dmASMa/AW6Syo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=fpefWz2OOWYhr17qhvXdJUHCSZlnkKeCwwFJI/K2lhYX9vBN17F0aiYUo7GSpMVxO ppO5B8VBOtTk7DNbQJ5QyFNUCsRHRxKbglk2Xy6mbDY77Xtby95tMV7tlnGqc4K+Oi qdJ6j9RZmslbWmQBmcu17X5PYsn/YKzDY6P4wXKgOLuZOiSqqx/sryvCAJLYsQPh+T hPJua80/8rB8rElwDvyNPo67qMQ6mhbVVJ/OlUcPjhclzwdporTHPm8jgMW1AaMU6l YS5rUv9aN/dSNW+MUNZixj6eYpamFotey5l0QVhzmJRafWDa99MNPsC7gnZ3xgmxeG Vn2WW1vgksAMA== Received: from thinkos.internal.efficios.com (unknown [IPv6:2606:6d00:100:4000:a253:d09e:90e7:323f]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4dWtlz3Z4MzbxM; Wed, 17 Dec 2025 20:45:35 -0500 (EST) From: Mathieu Desnoyers To: Boqun Feng , Joel Fernandes , "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Nicholas Piggin , Michael Ellerman , Greg Kroah-Hartman , Sebastian Andrzej Siewior , Will Deacon , Peter Zijlstra , Alan Stern , John Stultz , Neeraj Upadhyay , Linus Torvalds , Andrew Morton , Frederic Weisbecker , 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 , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev, Gary Guo , Nikita Popov , llvm@lists.linux.dev Subject: [RFC PATCH v4 1/4] compiler.h: Introduce ptr_eq() to preserve address dependency Date: Wed, 17 Dec 2025 20:45:28 -0500 Message-Id: <20251218014531.3793471-2-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20251218014531.3793471-1-mathieu.desnoyers@efficios.com> References: <20251218014531.3793471-1-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" Compiler CSE and SSA GVN optimizations can cause the address dependency of addresses returned by rcu_dereference to be lost when comparing those pointers with either constants or previously loaded pointers. Introduce ptr_eq() to compare two addresses while preserving the address dependencies for later use of the address. It should be used when comparing an address returned by rcu_dereference(). This is needed to prevent the compiler CSE and SSA GVN optimizations from using @a (or @b) in places where the source refers to @b (or @a) based on the fact that after the comparison, the two are known to be equal, which does not preserve address dependencies and allows the following misordering speculations: - If @b is a constant, the compiler can issue the loads which depend on @a before loading @a. - If @b is a register populated by a prior load, weakly-ordered CPUs can speculate loads which depend on @a before loading @a. The same logic applies with @a and @b swapped. Suggested-by: Linus Torvalds Suggested-by: Boqun Feng Signed-off-by: Mathieu Desnoyers Reviewed-by: Boqun Feng Reviewed-by: Joel Fernandes (Google) Tested-by: Joel Fernandes (Google) Acked-by: "Paul E. McKenney" Acked-by: Alan Stern 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 Cc: Nikita Popov Cc: llvm@lists.linux.dev --- Changes since v0: - Include feedback from Alan Stern. --- include/linux/compiler.h | 63 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 63 insertions(+) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 5b45ea7dff3e..c5ca3b54c112 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -163,6 +163,69 @@ void ftrace_likely_update(struct ftrace_likely_data *f= , int val, __asm__ ("" : "=3Dr" (var) : "0" (var)) #endif =20 +/* + * Compare two addresses while preserving the address dependencies for + * later use of the address. It should be used when comparing an address + * returned by rcu_dereference(). + * + * This is needed to prevent the compiler CSE and SSA GVN optimizations + * from using @a (or @b) in places where the source refers to @b (or @a) + * based on the fact that after the comparison, the two are known to be + * equal, which does not preserve address dependencies and allows the + * following misordering speculations: + * + * - If @b is a constant, the compiler can issue the loads which depend + * on @a before loading @a. + * - If @b is a register populated by a prior load, weakly-ordered + * CPUs can speculate loads which depend on @a before loading @a. + * + * The same logic applies with @a and @b swapped. + * + * Return value: true if pointers are equal, false otherwise. + * + * The compiler barrier() is ineffective at fixing this issue. It does + * not prevent the compiler CSE from losing the address dependency: + * + * int fct_2_volatile_barriers(void) + * { + * int *a, *b; + * + * do { + * a =3D READ_ONCE(p); + * asm volatile ("" : : : "memory"); + * b =3D READ_ONCE(p); + * } while (a !=3D b); + * asm volatile ("" : : : "memory"); <-- barrier() + * return *b; + * } + * + * With gcc 14.2 (arm64): + * + * fct_2_volatile_barriers: + * adrp x0, .LANCHOR0 + * add x0, x0, :lo12:.LANCHOR0 + * .L2: + * ldr x1, [x0] <-- x1 populated by first load. + * ldr x2, [x0] + * cmp x1, x2 + * bne .L2 + * ldr w0, [x1] <-- x1 is used for access which should depend= on b. + * ret + * + * On weakly-ordered architectures, this lets CPU speculation use the + * result from the first load to speculate "ldr w0, [x1]" before + * "ldr x2, [x0]". + * Based on the RCU documentation, the control dependency does not + * prevent the CPU from speculating loads. + */ +static __always_inline +int ptr_eq(const volatile void *a, const volatile void *b) +{ + OPTIMIZER_HIDE_VAR(a); + OPTIMIZER_HIDE_VAR(b); + return a =3D=3D b; +} + #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNT= ER__) =20 /** --=20 2.39.5