From patchwork Fri Sep 27 20:33:34 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Mathieu Desnoyers X-Patchwork-Id: 13814542 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 681EE1C1AAE; Fri, 27 Sep 2024 20:35:47 +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=1727469349; cv=none; b=e4mDTNL6B2NXlQ89JK1XPF73ZCixUbouM54LpumJgEwa2DwTiuGk7h2SN42aHEoCtJtvGgqaanPorbR7BfzsyDc3PNyjHx55oM4H3qk/A98YnuFljWnBtwTN3ydPShbBJRKVpEPOA9I+rukxW+wS3Z5rLtxqPJOolLKyGSvPRKo= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727469349; c=relaxed/simple; bh=XF5056z02azYzf5pHxQg2Q3bGDdaMsKYT1iCdrYtegI=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=ZkUPmwRTf3WMR6EMxMnlvbyGaXtTrHu97fwShWDwu0sTygrn2RJCmgoTrVvtPf66PWA/9wlhaVgoSF0SPKDskSIpm8GvSJQKAk9XmRi8uLGew0TJmY3pbLqtki2ahLoInxR8N31dtd4aQXyAt7HzFSbT/gRiKJTPCyZj03DR8hw= 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=grO9EOYR; 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="grO9EOYR" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1727469346; bh=XF5056z02azYzf5pHxQg2Q3bGDdaMsKYT1iCdrYtegI=; h=From:To:Cc:Subject:Date:From; b=grO9EOYRvZUdijajmWSbT98E/Ps5M4ViHfH/mvH8YiX31JYPgIKSMgWssllsRqNfa 2vLao3hEaT98OCAH5NQCJ1X5XvmUFBsd7otCVENm5FWDrB7QZShmK5l+mmrHTvhqxm d2UhwUZ/OmUEy05kt/Z2WsW+Z30tZnANBspG7m5Vf7V9JEwonIYF42Xfk4Dc5fI+x/ Z2BF6l8osnzL5Ec6PwUyZ8T+26yFvuCMVymuzhiBWUYxJB3quunRtZ6lwBZbBp5sy/ 9eliKO58HQ6dKXPGDrcS2kO9tFTRpmdGayZpv01NDB9/P4tQCJ37GrBTQkfThLrfzc JRiQWOcNQ4DEg== Received: from thinkos.internal.efficios.com (unknown [IPv6:2606:6d00:100:4000:cacb:9855:de1f:ded2]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XFj0L24bPz1RhW; Fri, 27 Sep 2024 16:35:46 -0400 (EDT) From: Mathieu Desnoyers To: Linus Torvalds Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Boqun Feng , Greg Kroah-Hartman , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , Peter Zijlstra , 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 , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev Subject: [RFC PATCH] compiler.h: Introduce ptr_eq() to preserve address dependency Date: Fri, 27 Sep 2024 16:33:34 -0400 Message-Id: <20240927203334.976821-1-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.2 Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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 replacing the registers holding @a or @b based on their equality, 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. 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 = READ_ONCE(p); asm volatile ("" : : : "memory"); b = READ_ONCE(p); } while (a != 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. Suggested-by: Linus Torvalds Suggested-by: Boqun Feng 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: rcu@vger.kernel.org Cc: linux-mm@kvack.org Cc: lkmm@lists.linux.dev Reviewed-by: Boqun Feng Acked-by: Paul E. McKenney --- include/linux/compiler.h | 62 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 62 insertions(+) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 2df665fa2964..f26705c267e8 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -186,6 +186,68 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, __asm__ ("" : "=r" (var) : "0" (var)) #endif +/* + * 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 replacing the registers holding @a or @b based on their + * equality, 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 = READ_ONCE(p); + * asm volatile ("" : : : "memory"); + * b = READ_ONCE(p); + * } while (a != 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 == b; +} + #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__) /**