From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 0A77645DF6C for ; Thu, 10 Sep 2026 10:56:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789037820; cv=none; b=WN8ZJ57mF4fr8r7gWTCm3KvvOmETsCkzSWnH4/dLyNFfBls3uUFUn6vdpKiCYNnvmh3fBejQrhcZioLJ96KLfQbEnVil695tZPrxMX5QpOrvAmwl7eEeHPCRJCpOuLZgcRu4uXtBSRUyET/eZqfU6Y9hG5mZz8wbfXdW23HGRMA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789037820; c=relaxed/simple; bh=0XVud+aFs7SjXyrBbSbz9+RlhMfbfwciyLfpZ2J7E6k=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=bNHma+Hibyf/siRsxWIgQjsVqm4mjTnlSSyRVXbv/a2Hi8JrsVkYEKYUAhZcrkSGaw0UgZwx9FIpj6xoQ6FAXoekrj7X/4wqrqBogRNJytGXnALa+ivNoUNaZDuofIwg8OkcLmUD0JApYJ9oGMnQtCMPI7u9qaRFEuiEp3pN2Wc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QugRe20W; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="QugRe20W" 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 Cc: linux-arm-kernel@lists.infradead.org, ada.coupriediaz@arm.com, ardb@kernel.org, catalin.marinas@arm.com, cl@gentwo.org, david@kernel.org, david.laight.linux@gmail.com, james.morse@arm.com, maz@kernel.org, peterz@infradead.org, ruanjinjie@huawei.com, ryan.roberts@arm.com, stable@vger.kernel.org, usama.anjum@arm.com, vladimir.murzin@arm.com, will@kernel.org, yang@os.amperecomputing.com 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> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260908151741.394589-2-mark.rutland@arm.com> 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