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 X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B81D8C43603 for ; Mon, 16 Dec 2019 09:45:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8A6CD20725 for ; Mon, 16 Dec 2019 09:45:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576489531; bh=ID6AxVJaN/XZPaFJDNB6plYUfGK4flJOFGIZw/PCpfI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=mFxL40V9XPMsYC0E5PWoNl2UWy+4Ui1+4NMgRW1gjkdFC3PCgSOlmLIwxN6XdIlKy +dx+xepWndVfQqV9I7ZEgUKzjwWplmVacbsmbMCM4lozoy/u5+b0EC5qsFCui51Q9a 14Z0FYwqWaLwf76/HFoGVzUH8yE+9c9lcTYPCt40= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727070AbfLPJpb (ORCPT ); Mon, 16 Dec 2019 04:45:31 -0500 Received: from mail.kernel.org ([198.145.29.99]:49274 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726937AbfLPJpa (ORCPT ); Mon, 16 Dec 2019 04:45:30 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D319320665; Mon, 16 Dec 2019 09:45:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576489529; bh=ID6AxVJaN/XZPaFJDNB6plYUfGK4flJOFGIZw/PCpfI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=usiK59CUF4wvbn/67RYEOn0Dl40MAPAvz11JWIwlYU8+v1B/eA6RgcQdph0DQoTse Uj9p2alU1Ex+bT22CAN1SPQ55yltS4CATkUF8CVhrxuLPw1v+4YAB2nPL5OmW/dDkm qn0CPviLJpyHQ7NqWfK4Ff2LapqUV9TjUR1xFzkk= Date: Mon, 16 Dec 2019 09:45:24 +0000 From: Will Deacon To: Steven Rostedt Cc: Sasha Levin , "kernelci.org bot" , Ard Biesheuvel , Kevin Hilman , Will Deacon , linux-kernel@vger.kernel.org, stable@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH AUTOSEL 4.19 031/219] arm64: preempt: Fix big-endian when checking preempt count in assembly Message-ID: <20191216094523.GA9938@willie-the-truck> References: <20191122054911.1750-1-sashal@kernel.org> <20191122054911.1750-24-sashal@kernel.org> <20191214021403.GA1357@home.goodmis.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191214021403.GA1357@home.goodmis.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Fri, Dec 13, 2019 at 09:14:03PM -0500, Steven Rostedt wrote: > On Fri, Nov 22, 2019 at 12:46:03AM -0500, Sasha Levin wrote: > > From: Will Deacon > > > > [ Upstream commit 7faa313f05cad184e8b17750f0cbe5216ac6debb ] > > > > Commit 396244692232 ("arm64: preempt: Provide our own implementation of > > asm/preempt.h") extended the preempt count field in struct thread_info > > to 64 bits, so that it consists of a 32-bit count plus a 32-bit flag > > indicating whether or not the current task needs rescheduling. > > > > Whilst the asm-offsets definition of TSK_TI_PREEMPT was updated to point > > to this new field, the assembly usage was left untouched meaning that a > > 32-bit load from TSK_TI_PREEMPT on a big-endian machine actually returns > > the reschedule flag instead of the count. > > > > Whilst we could fix this by pointing TSK_TI_PREEMPT at the count field, > > we're actually better off reworking the two assembly users so that they > > operate on the whole 64-bit value in favour of inspecting the thread > > flags separately in order to determine whether a reschedule is needed. > > > > Acked-by: Ard Biesheuvel > > Reported-by: "kernelci.org bot" > > Tested-by: Kevin Hilman > > Signed-off-by: Will Deacon > > Signed-off-by: Sasha Levin > > --- > > arch/arm64/include/asm/assembler.h | 8 +++----- > > arch/arm64/kernel/entry.S | 6 ++---- > > 2 files changed, 5 insertions(+), 9 deletions(-) > > > > diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h > > index 5a97ac8531682..0c100506a29aa 100644 > > --- a/arch/arm64/include/asm/assembler.h > > +++ b/arch/arm64/include/asm/assembler.h > > @@ -683,11 +683,9 @@ USER(\label, ic ivau, \tmp2) // invalidate I line PoU > > .macro if_will_cond_yield_neon > > #ifdef CONFIG_PREEMPT > > get_thread_info x0 > > - ldr w1, [x0, #TSK_TI_PREEMPT] > > - ldr x0, [x0, #TSK_TI_FLAGS] > > - cmp w1, #PREEMPT_DISABLE_OFFSET > > - csel x0, x0, xzr, eq > > - tbnz x0, #TIF_NEED_RESCHED, .Lyield_\@ // needs rescheduling? > > + ldr x0, [x0, #TSK_TI_PREEMPT] > > + sub x0, x0, #PREEMPT_DISABLE_OFFSET > > + cbz x0, .Lyield_\@ > > /* fall through to endif_yield_neon */ > > .subsection 1 > > .Lyield_\@ : > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > > index 5f800384cb9a8..bb68323530458 100644 > > --- a/arch/arm64/kernel/entry.S > > +++ b/arch/arm64/kernel/entry.S > > @@ -622,10 +622,8 @@ el1_irq: > > irq_handler > > > > #ifdef CONFIG_PREEMPT > > - ldr w24, [tsk, #TSK_TI_PREEMPT] // get preempt count > > - cbnz w24, 1f // preempt count != 0 > > - ldr x0, [tsk, #TSK_TI_FLAGS] // get flags > > - tbz x0, #TIF_NEED_RESCHED, 1f // needs rescheduling? > > + ldr x24, [tsk, #TSK_TI_PREEMPT] // get preempt count > > + cbnz x24, 1f // preempt count != 0 > > bl el1_preempt > > While updating 4.19-rt, I stumbled on this change to arm64 backport. And was > confused by it, but looking deeper, this is something that breaks without > having 396244692232f ("arm64: preempt: Provide our own implementation of > asm/preempt.h"). > > That commit inverts the TIF_NEED_RESCHED meaning where set means we don't need > to resched, and clear means we need to resched. This way we can combine the > preempt count with the need resched flag test as they share the same 64bit > word. A 0 means we need to preempt (as NEED_RESCHED being zero means we need > to resched, and this also means preempt_count is zero). If the > TIF_NEED_RESCHED bit is set, that means we don't need to resched, and if > preempt count is something other than zero, we don't need to resched, and > since those two are together by commit 396244692232f, we can just test > #TSK_TI_PREEMPT. But because that commit does not exist in 4.19, we can't > remove the TIF_NEED_RESCHED check, that this backport does, and then breaks > the kernel! Yup, without 396244692232 this commit makes no sense. That's why I didn't CC stable or add a Fixes tag :( Will