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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 4D98CC4360F for ; Thu, 4 Apr 2019 10:42:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 24D2720855 for ; Thu, 4 Apr 2019 10:42:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729536AbfDDKmY (ORCPT ); Thu, 4 Apr 2019 06:42:24 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:57796 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729118AbfDDKmX (ORCPT ); Thu, 4 Apr 2019 06:42:23 -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 8CB52A78; Thu, 4 Apr 2019 03:42:22 -0700 (PDT) Received: from [10.1.196.93] (en101.cambridge.arm.com [10.1.196.93]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D8DB13F557; Thu, 4 Apr 2019 03:42:20 -0700 (PDT) Subject: Re: [PATCH v2 1/2] perf/arm-cci: Remove broken race mitigation To: robin.murphy@arm.com, will.deacon@arm.com Cc: mark.rutland@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, bigeasy@linutronix.de, peterz@infradead.org, clabbe.montjoie@gmail.com, Meng.Li@windriver.com References: From: Suzuki K Poulose Message-ID: Date: Thu, 4 Apr 2019 11:42:19 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/04/2019 18:10, Robin Murphy wrote: > Uncore PMU drivers face an awkward cyclic dependency wherein: > > - They have to pick a valid online CPU to associate with before > registering the PMU device, since it will get exposed to userspace > immediately. > - The PMU registration has to be be at least partly complete before > hotplug events can be handled, since trying to migrate an > uninitialised context would be bad. > - The hotplug handler has to be ready as soon as a CPU is chosen, lest > it go offline without the user-visible cpumask value getting updated. > > The arm-cci driver has tried to solve this by using get_cpu() to pick > the current CPU and prevent it from disappearing while both > registrations are performed, but that results in taking mutexes with > preemption disabled, which makes certain configurations very unhappy: > > [ 1.983337] BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:2004 > [ 1.983340] in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: swapper/0 > [ 1.983342] Preemption disabled at: > [ 1.983353] [] cci_pmu_probe+0x1dc/0x488 > [ 1.983360] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.20-rt8-yocto-preempt-rt #1 > [ 1.983362] Hardware name: ZynqMP ZCU102 Rev1.0 (DT) > [ 1.983364] Call trace: > [ 1.983369] dump_backtrace+0x0/0x158 > [ 1.983372] show_stack+0x24/0x30 > [ 1.983378] dump_stack+0x80/0xa4 > [ 1.983383] ___might_sleep+0x138/0x160 > [ 1.983386] __might_sleep+0x58/0x90 > [ 1.983391] __rt_mutex_lock_state+0x30/0xc0 > [ 1.983395] _mutex_lock+0x24/0x30 > [ 1.983400] perf_pmu_register+0x2c/0x388 > [ 1.983404] cci_pmu_probe+0x2bc/0x488 > [ 1.983409] platform_drv_probe+0x58/0xa8 > > It is not feasible to resolve all the possible races outside of the perf > core itself, so address the immediate bug by following the example of > nearly every other PMU driver and not even trying to do so. Registering > the hotplug notifier first should minimise the window in which things > can go wrong, so that's about as much as we can reasonably do here. This > also revealed an additional race in assigning the global pointer too > late relative to the hotplug notifier, which gets fixed in the process. > > Reported-by: "Li, Meng" > Reported-by: Corentin Labbe > Signed-off-by: Robin Murphy Thanks for fixing this. Reviewed-by: Suzuki K Poulose