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=-14.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 72116C4338F for ; Tue, 10 Aug 2021 09:28:53 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 1F3D160EE9 for ; Tue, 10 Aug 2021 09:28:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 1F3D160EE9 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:Message-ID:Date:References :In-Reply-To:Subject:Cc:To:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=BX6KhKwY59TYs9BoWbYO7P8RGvZlMqZCfntDS5jevUY=; b=XqyxvQOhcVyHQ9 eiN0qu08EY1bYIOkHOULXQ3v5rFWFHSkGo1bXOcGtqzyzzqLU9ipDKnk8zwYy9FycB8mutFTuDGPw 0awDdlnK8r43NLHlCPnX7iSWqOXkgLVrbqjJh03CB6zJax5jUigh5jJgK5yU4Ixtjn3Xla50PSQye +e3cdsvKIe04xpzvH4LEau7BMDMkUXmCADgOsZzMiMr4uURvkNs4qGml6N7BXGFJzEwJP94IIB/1V pkRl4mX0PKZQIBhZnLGR32T+WMLk9N+EdYboOcOX/0+5gVwzpKCMlXugOvNs8h3ptkhvvo5bOWLew w4RGsHYGkUEubBOnizzQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1mDO1n-003GyF-HX; Tue, 10 Aug 2021 09:26:31 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1mDO1j-003Gxa-Dv for linux-arm-kernel@lists.infradead.org; Tue, 10 Aug 2021 09:26:28 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9FE7DD6E; Tue, 10 Aug 2021 02:26:22 -0700 (PDT) Received: from e113632-lin (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BFF863F718; Tue, 10 Aug 2021 02:26:19 -0700 (PDT) From: Valentin Schneider To: Boqun Feng Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org, linux-rt-users@vger.kernel.org, Catalin Marinas , Will Deacon , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Steven Rostedt , Daniel Bristot de Oliveira , Sebastian Andrzej Siewior , "Paul E. McKenney" , Frederic Weisbecker , Josh Triplett , Mathieu Desnoyers , Davidlohr Bueso , Lai Jiangshan , Joel Fernandes , Anshuman Khandual , Vincenzo Frascino , Steven Price , Ard Biesheuvel Subject: Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe() In-Reply-To: References: <20210807005807.1083943-1-valentin.schneider@arm.com> <20210807005807.1083943-3-valentin.schneider@arm.com> Date: Tue, 10 Aug 2021 10:26:15 +0100 Message-ID: <871r71abwo.mognet@arm.com> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210810_022627_609698_4F971544 X-CRM114-Status: GOOD ( 19.81 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi, On 10/08/21 10:42, Boqun Feng wrote: > Hi, > > On Sat, Aug 07, 2021 at 01:58:05AM +0100, Valentin Schneider wrote: >> Some areas use preempt_disable() + preempt_enable() to safely access >> per-CPU data. The PREEMPT_RT folks have shown this can also be done by >> keeping preemption enabled and instead disabling migration (and acquiring a >> sleepable lock, if relevant). >> >> Introduce a helper which checks whether the current task can safely access >> per-CPU data, IOW if the task's context guarantees the accesses will target >> a single CPU. This accounts for preemption, CPU affinity, and migrate >> disable - note that the CPU affinity check also mandates the presence of >> PF_NO_SETAFFINITY, as otherwise userspace could concurrently render the >> upcoming per-CPU access(es) unsafe. >> >> Signed-off-by: Valentin Schneider >> --- >> include/linux/sched.h | 10 ++++++++++ >> 1 file changed, 10 insertions(+) >> >> diff --git a/include/linux/sched.h b/include/linux/sched.h >> index debc960f41e3..b77d65f677f6 100644 >> --- a/include/linux/sched.h >> +++ b/include/linux/sched.h >> @@ -1715,6 +1715,16 @@ static inline bool is_percpu_thread(void) >> #endif >> } >> >> +/* Is the current task guaranteed not to be migrated elsewhere? */ >> +static inline bool is_pcpu_safe(void) >> +{ >> +#ifdef CONFIG_SMP >> + return !preemptible() || is_percpu_thread() || current->migration_disabled; >> +#else >> + return true; >> +#endif >> +} > > I wonder whether the following can happen, say thread A is a worker > thread for CPU 1, so it has the flag PF_NO_SETAFFINITY set. > > { percpu variable X on CPU 2 is initially 0 } > > thread A > ======== > > > if (is_pcpu_safe()) { // nr_cpus_allowed == 1, so return true. > > > unbinder_workers(1); // A->cpus_mask becomes cpu_possible_mask > > __this_cpu_inc(X); > tmp = X; // tmp == 0 > > > this_cpu_inc(X); // X becomes 1 > > X = tmp + 1; // race! > } > > if so, then is_percpu_thread() doesn't indicate is_pcpu_safe()? > You're absolutely right. migrate_disable() protects the thread against being migrated due to hotplug, but pure CPU affinity doesn't at all. kthread_is_per_cpu() doesn't work either, because parking is not the only approach to hotplug for those (e.g. per-CPU workqueue threads unbind themselves on hotplug, as in your example). One could hold cpus_read_lock(), but I don't see much point here. So that has to be return !preemptible() || current->migration_disabled; Thanks! _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel