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, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS 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 60D18C282C4 for ; Tue, 12 Feb 2019 09:15:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 384C821773 for ; Tue, 12 Feb 2019 09:15:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728716AbfBLJPU (ORCPT ); Tue, 12 Feb 2019 04:15:20 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:33820 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726325AbfBLJPT (ORCPT ); Tue, 12 Feb 2019 04:15:19 -0500 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 EF72DEBD; Tue, 12 Feb 2019 01:15:18 -0800 (PST) Received: from [10.1.197.45] (e112298-lin.cambridge.arm.com [10.1.197.45]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5A6843F557; Tue, 12 Feb 2019 01:15:16 -0800 (PST) Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region To: Peter Zijlstra , Ingo Molnar Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mingo@redhat.com, catalin.marinas@arm.com, will.deacon@arm.com, james.morse@arm.com, hpa@zytor.com, valentin.schneider@arm.com References: <1547560709-56207-1-git-send-email-julien.thierry@arm.com> <1547560709-56207-4-git-send-email-julien.thierry@arm.com> <20190211134527.GA121589@gmail.com> <20190211135159.GC32511@hirez.programming.kicks-ass.net> From: Julien Thierry Message-ID: Date: Tue, 12 Feb 2019 09:15:13 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190211135159.GC32511@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 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 11/02/2019 13:51, Peter Zijlstra wrote: > On Mon, Feb 11, 2019 at 02:45:27PM +0100, Ingo Molnar wrote: >>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >>> index a674c7db..b1bb7e9 100644 >>> --- a/kernel/sched/core.c >>> +++ b/kernel/sched/core.c >>> @@ -3289,6 +3289,14 @@ static inline void schedule_debug(struct task_struct *prev) >>> __schedule_bug(prev); >>> preempt_count_set(PREEMPT_DISABLED); >>> } >>> + >>> + if (IS_ENABLED(CONFIG_DEBUG_UACCESS_SLEEP) && >>> + unlikely(unsafe_user_region_active())) { >>> + printk(KERN_ERR "BUG: scheduling while user_access enabled: %s/%d/0x%08x\n", >>> + prev->comm, prev->pid, preempt_count()); >>> + dump_stack(); >>> + } >>> + >>> rcu_sleep_check(); >>> >>> profile_hit(SCHED_PROFILING, __builtin_return_address(0)); >>> @@ -6151,6 +6159,20 @@ void ___might_sleep(const char *file, int line, int preempt_offset) >>> EXPORT_SYMBOL(___might_sleep); >>> #endif >>> >>> +#ifdef CONFIG_DEBUG_UACCESS_SLEEP >>> +void __might_resched(const char *file, int line) >>> +{ >>> + if (!unsafe_user_region_active()) >>> + return; >> >> Could you please more clearly explain why you want/need an exception from >> the __might_resched() debug warning? So, the scenarios I'm trying to avoid are of the following flavour: if (user_access_begin(ptr, size)) { [...] // Calling a function that might call schedule() [...] user_access_end(); } The thing is, as I understand, not all function that call schedule() are annotated with might_resched(), and on the other hand, not every time we call a function that might_resched() will it call schedule(). Now with Peter's remark I think I might have been overzealous. > > In specific; how is the addition in schedule_debug() not triggering on > PREEMPT=y kernels? > > If code is preemptible, you can (get) schedule(d). If it is not > preemptible; you do not need these additional tests. > Yes that sounds right, might_resched() only potentially reschedules if in a suitable context, so best case I issue two warnings, worst case I actually be warn when the caller took care to disable preemption or interrupts before calling a might_resched(). I guess I got a bit confused with might_sleep() which is "if you call this in the wrong context I warn" whereas might_resched() is just "if you call this in preemptible context, lets resched". I guess I'll drop the might_resched() part of this patch if that sounds alright. Thanks, -- Julien Thierry