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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 6C265C79FB9 for ; Thu, 10 Sep 2026 10:56:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Cc:List-Subscribe: List-Help:List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To: Content-Type:MIME-Version:References:Message-ID:Subject:To:From:Date:Reply-To :Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=vy0obgvg0nf8qGQKnYvGOGI82XsQ6j8jix882VNn/Nc=; b=xkTHeaqRklxw86hhI7VNUslyEE Z8I6I104yeg6YEt1Z9QxnUsWhcppPrt/32UzdyPUoeu+tqXeFAXOo4zF/9aiWu+Ew3H8N+ijmGJ8w jUjHFIspM41347t1zxcxF5l69TElz6XR/VwSG8+ArGmf7Pcc4F8Fv9WPuzBRjMX0WwG2c6EC6PyRR XeD4wSksp+9esbYfs9RnZEfmCbq82Z8Y8Di9xdIqtuXcHrohybxAjA6Wqq8rlu0jMLARgHQ9/4fVd fLGEP2viz+i5M9ff1WzPIVhkEB9xXNHu/7P/z/WRgMVXXwmBJxllGfnUvXsX96lwENRLMvQ1gIKLW OfS3KEzw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x4cSZ-0000000E7gd-472z; Thu, 10 Sep 2026 10:56:51 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x4cSZ-0000000E7gQ-0uMT for linux-arm-kernel@lists.infradead.org; Thu, 10 Sep 2026 10:56:51 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id E09804128A; Thu, 10 Sep 2026 10:56:50 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2EB9B1F000FF; Thu, 10 Sep 2026 10:56:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789037810; bh=vy0obgvg0nf8qGQKnYvGOGI82XsQ6j8jix882VNn/Nc=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=QugRe20W6g3MjyJ+mgnoAlyfxdfcW8qlXrm53ytBcd1jTs5KN6YljfasGpJHQ0FQC lJJ8Iz4hQN99WZlHJTKqfF/kSqZzxDE8ON4Tn0fE32XSyUBMPn1rR8LgiuOW2M878K IUqWJIUeZGXkk0wxvGn871PZGtXDYfzvQIs+jTnvvUk30CtHJy9Md9Lwyocou+5qj7 7Ztlfmjx6Ut7DVwCSWsi6zclwPdLt9a7YrjX+Ng831FVWh59qswdmYF6ayEBsnwjQ8 wA7QPytJP5GbfpiXZ+cnkByeHec4QR/zQYz07ap+c3GctQRu/thP7omkDpUpFa4OUL bvSon05EQ/nWw== Date: Thu, 10 Sep 2026 11:56:43 +0100 From: "Lorenzo Stoakes (ARM)" To: Mark Rutland Subject: Re: [PATCH v4 01/21] arm64: percpu: Fix this_cpu_write() casting Message-ID: References: <20260908151741.394589-1-mark.rutland@arm.com> <20260908151741.394589-2-mark.rutland@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260908151741.394589-2-mark.rutland@arm.com> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: vladimir.murzin@arm.com, ryan.roberts@arm.com, usama.anjum@arm.com, peterz@infradead.org, catalin.marinas@arm.com, david.laight.linux@gmail.com, stable@vger.kernel.org, ruanjinjie@huawei.com, james.morse@arm.com, yang@os.amperecomputing.com, cl@gentwo.org, maz@kernel.org, david@kernel.org, will@kernel.org, ardb@kernel.org, linux-arm-kernel@lists.infradead.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Sep 08, 2026 at 04:17:21PM +0100, Mark Rutland wrote: > The arm64 implementation of this_cpu_write() casts 'val' to unsigned > long. This is necessary to handle cases where 'val' is a pointer type, > and to avoid spurious compiler warnings for the (unreachable!) cases > where the pointer type would be cast to a smaller integer type. > > Unfortunately, the cast is applied to 'val' rather than '(val)', which > won't always generate the expected value when 'val' is an expression. > > For example, for this_cpu_write(pcp, zero - 1), where 'pcp' is a u64 and > 'zero' is a u32: > > * 'zero' ===> (u32) 0x00000000 > * 'zero - 1' ===> (u32) 0xffffffff > * '(unsigned long)zero - 1' ===> (u64) 0xffffffffffffffff > * '(unsigned long)(zero - 1)' ===> (u64) 0x00000000ffffffff > > Fix this by adding brackets around 'val' > > The bug described above can be seen from the disassembly of the > following test code: > > | void this_cpu_write_zero_minus_1(u64 __percpu *pcp) > | { > | u32 zero = 0; > | this_cpu_write(*pcp, zero - 1); > | } > | > | void this_cpu_write_zero_minus_1_brackets(u64 __percpu *pcp) > | { > | u32 zero = 0; > | this_cpu_write(*pcp, (zero - 1)); > | } > > Generated code before this patch: > > | : > | paciasp > | stp x29, x30, [sp, #-16]! > | mrs x1, sp_el0 > | mov x29, sp > | ldr w2, [x1, #8] > | add w2, w2, #0x1 > | str w2, [x1, #8] > | mov x3, #0xffffffffffffffff > | mrs x2, tpidr_el1 > | str x3, [x0, x2] > | ldr x0, [x1, #8] > | add x0, x0, x3 > | str w0, [x1, #8] > | cbz x0, 1f > | ldr x0, [x1, #8] > | cbnz x0, 2f > | 1: bl preempt_schedule_notrace > | 2: ldp x29, x30, [sp], #16 > | autiasp > | ret > | > | : > | paciasp > | stp x29, x30, [sp, #-16]! > | mrs x1, sp_el0 > | mov x29, sp > | ldr w2, [x1, #8] > | add w2, w2, #0x1 > | str w2, [x1, #8] > | mov x3, #0xffffffff > | mrs x2, tpidr_el1 > | str x3, [x0, x2] > | ldr x0, [x1, #8] > | sub x0, x0, #0x1 > | str w0, [x1, #8] > | cbz x0, 1f > | ldr x0, [x1, #8] > | cbnz x0, 2f > | 1: bl preempt_schedule_notrace > | 2: ldp x29, x30, [sp], #16 > | autiasp > | ret > > Generated code after this patch: > > | : > | paciasp > | stp x29, x30, [sp, #-16]! > | mrs x1, sp_el0 > | mov x29, sp > | ldr w2, [x1, #8] > | add w2, w2, #0x1 > | str w2, [x1, #8] > | mov x3, #0xffffffff > | mrs x2, tpidr_el1 > | str x3, [x0, x2] > | ldr x0, [x1, #8] > | sub x0, x0, #0x1 > | str w0, [x1, #8] > | cbz x0, 1f > | ldr x0, [x1, #8] > | cbnz x0, 2f > | 1: bl preempt_schedule_notrace > | 2: ldp x29, x30, [sp], #16 > | autiasp > | ret > | > | : > | b this_cpu_write_zero_minus_1 > > Fixes: 959bf2fd03b5 ("arm64: percpu: Rewrite per-cpu ops to allow use of LSE atomics") > Reported-by: David Laight > Signed-off-by: Mark Rutland Ah the fun of arithmetic conversions in C :) LGTM so: Reviewed-by: Lorenzo Stoakes (ARM) > Reviewed-by: David Laight > Reviewed-by: Jinjie Ruan > Tested-by: Muhammad Usama Anjum > Acked-by: Christopher Lameter (Ampere) > Cc: Ada Couprie Diaz > Cc: Ard Biesheuvel > Cc: Catalin Marinas > Cc: James Morse > Cc: Marc Zyngier > Cc: Peter Zijlstra > Cc: Vladimir Murzin > Cc: Will Deacon > Cc: Yang Shi > Cc: stable@vger.kernel.org > --- > arch/arm64/include/asm/percpu.h | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/include/asm/percpu.h b/arch/arm64/include/asm/percpu.h > index b57b2bb009677..63bbfd4944a37 100644 > --- a/arch/arm64/include/asm/percpu.h > +++ b/arch/arm64/include/asm/percpu.h > @@ -179,13 +179,13 @@ PERCPU_RET_OP(add, add, ldadd) > _pcp_protect_return(__percpu_read_64, pcp) > > #define this_cpu_write_1(pcp, val) \ > - _pcp_protect(__percpu_write_8, pcp, (unsigned long)val) > + _pcp_protect(__percpu_write_8, pcp, (unsigned long)(val)) > #define this_cpu_write_2(pcp, val) \ > - _pcp_protect(__percpu_write_16, pcp, (unsigned long)val) > + _pcp_protect(__percpu_write_16, pcp, (unsigned long)(val)) > #define this_cpu_write_4(pcp, val) \ > - _pcp_protect(__percpu_write_32, pcp, (unsigned long)val) > + _pcp_protect(__percpu_write_32, pcp, (unsigned long)(val)) > #define this_cpu_write_8(pcp, val) \ > - _pcp_protect(__percpu_write_64, pcp, (unsigned long)val) > + _pcp_protect(__percpu_write_64, pcp, (unsigned long)(val)) > > #define this_cpu_add_1(pcp, val) \ > _pcp_protect(__percpu_add_case_8, pcp, val) > -- > 2.30.2 > -- Cheers, Lorenzo