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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_MUTT 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 A7405C28CC0 for ; Thu, 30 May 2019 12:01:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 79AFF258A9 for ; Thu, 30 May 2019 12:01:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727218AbfE3MBe (ORCPT ); Thu, 30 May 2019 08:01:34 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:35048 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726997AbfE3MBe (ORCPT ); Thu, 30 May 2019 08:01:34 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E2154374; Thu, 30 May 2019 05:01:33 -0700 (PDT) Received: from fuggles.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 2086D3F5AF; Thu, 30 May 2019 05:01:31 -0700 (PDT) Date: Thu, 30 May 2019 13:01:29 +0100 From: Will Deacon To: Julien Grall Cc: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, linux-arm-kernel@lists.infradead.org, tglx@linutronix.de, rostedt@goodmis.org, bigeasy@linutronix.de, suzuki.poulose@arm.com, catalin.marinas@arm.com, dave.martin@arm.com Subject: Re: [PATCH] arm64/cpufeature: Convert hook_lock to raw_spin_lock_t in cpu_enable_ssbs() Message-ID: <20190530120129.GD13751@fuggles.cambridge.arm.com> References: <20190530113058.1988-1-julien.grall@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190530113058.1988-1-julien.grall@arm.com> User-Agent: Mutt/1.11.1+86 (6f28e57d73f2) () Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 30, 2019 at 12:30:58PM +0100, Julien Grall wrote: > cpu_enable_ssbs() is called via stop_machine() as part of the cpu_enable > callback. A spin lock is used to ensure the hook is registered before > the rest of the callback is executed. > > On -RT spin_lock() may sleep. However, all the callees in stop_machine() > are expected to not sleep. Therefore a raw_spin_lock() is required here. > > Given this is already done under stop_machine() and the work done under > the lock is quite small, the latency should not increase too much. > > Signed-off-by: Julien Grall > > --- > > It was noticed when looking at the current use of spin_lock in > arch/arm64. I don't have a platform calling that callback, so I have > hacked the code to reproduce the error and check it is now fixed. > --- > arch/arm64/kernel/cpufeature.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c > index ca27e08e3d8a..2a7159fda3ce 100644 > --- a/arch/arm64/kernel/cpufeature.c > +++ b/arch/arm64/kernel/cpufeature.c > @@ -1194,14 +1194,14 @@ static struct undef_hook ssbs_emulation_hook = { > static void cpu_enable_ssbs(const struct arm64_cpu_capabilities *__unused) > { > static bool undef_hook_registered = false; > - static DEFINE_SPINLOCK(hook_lock); > + static DEFINE_RAW_SPINLOCK(hook_lock); > > - spin_lock(&hook_lock); > + raw_spin_lock(&hook_lock); > if (!undef_hook_registered) { > register_undef_hook(&ssbs_emulation_hook); > undef_hook_registered = true; > } > - spin_unlock(&hook_lock); > + raw_spin_unlock(&hook_lock); Makes sense to me. We could probably avoid the lock entirely if we wanted to (via atomic_dec_if_positive), but I'm not sure it's really worth it. Will