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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 92FC5C433EF for ; Tue, 24 May 2022 16:39:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238704AbiEXQjm (ORCPT ); Tue, 24 May 2022 12:39:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41558 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233418AbiEXQjk (ORCPT ); Tue, 24 May 2022 12:39:40 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 83698E023 for ; Tue, 24 May 2022 09:39:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1653410377; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=fBeQ/9otl7euJ3gJ516ZBteQsGcd+PSie7gaIlgj0ZI=; b=IXo6ADJHu4DSaH+FXmCLrDTSMbiAYrtjsIsTx3Emx02jaTvLwpc5ZZzomQyDyc9XzyKWuF 7TZKakg74aylgvCKLtUsi9xn8yoJ16DaFRcnE9LgRoL/vb+y0JUZgm0pgh0bezPMN+kgle UEgVST26d6Pd90UO8OtuKJpp9qML6Uo= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-150-iQYTbkV1PEye8_HbwoHjvw-1; Tue, 24 May 2022 12:39:35 -0400 X-MC-Unique: iQYTbkV1PEye8_HbwoHjvw-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 2C543858EFE; Tue, 24 May 2022 16:39:35 +0000 (UTC) Received: from lorien.usersys.redhat.com (unknown [10.22.33.52]) by smtp.corp.redhat.com (Postfix) with ESMTPS id F0DBC1410DD5; Tue, 24 May 2022 16:39:34 +0000 (UTC) Date: Tue, 24 May 2022 12:39:33 -0400 From: Phil Auld To: Valentin Schneider Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra Subject: Re: [PATCH] cpuhp: make target_store() a nop when target == state Message-ID: References: <20220523144728.32414-1-pauld@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.85 on 10.11.54.7 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 24, 2022 at 04:11:51PM +0100 Valentin Schneider wrote: > On 23/05/22 10:47, Phil Auld wrote: > > writing the current state back into hotplug/target calls cpu_down() > > which will set cpu dying even when it isn't and then nothing will > > ever clear it. A stress test that reads values and writes them back > > for all cpu device files in sysfs will trigger the BUG() in > > select_fallback_rq once all cpus are marked as dying. > > > > kernel/cpu.c::target_store() > > ... > > if (st->state < target) > > ret = cpu_up(dev->id, target); > > else > > ret = cpu_down(dev->id, target); > > > > cpu_down() -> cpu_set_state() > > bool bringup = st->state < target; > > ... > > if (cpu_dying(cpu) != !bringup) > > set_cpu_dying(cpu, !bringup); > > > > Make this safe by catching the case where target == state > > and bailing early. > > > > Signed-off-by: Phil Auld > > --- > > > > Yeah, I know... don't do that. But it's still messy. > > > > !< != > > > > > kernel/cpu.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/kernel/cpu.c b/kernel/cpu.c > > index d0a9aa0b42e8..8a71b1149c60 100644 > > --- a/kernel/cpu.c > > +++ b/kernel/cpu.c > > @@ -2302,6 +2302,9 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr, > > return -EINVAL; > > #endif > > > > + if (target == st->state) > > + return count; > > + > > The current checks are against static boundaries, this has to compare > against st->state - AFAICT this could race with another hotplug operation > to the same CPU, e.g. > > CPU42.cpuhp_state > ->state == CPUHP_AP_SCHED_STARTING > ->target == CPUHP_ONLINE > > > > CPU42.cpuhp_state == CPUHP_ONLINE > > > What I'm trying to fix is not a race. It's just bogus logic. There is an assumption here that !< means > which is just not true. This potential race seems orthogonal and not even effected one way or the other by this code change, right? I could not convince myself that the check I added needed to be under the locks because returning success when the state is already reporting what you asked for seems harmless. > > _cpu_up() has: > > /* > * The caller of cpu_up() might have raced with another > * caller. Nothing to do. > */ > if (st->state >= target) > goto out; > > Looks like we want an equivalent in _cpu_down(), what do you think? Maybe. I still think that > > if (st->state < target) > > ret = cpu_up(dev->id, target); > > else > > ret = cpu_down(dev->id, target); is not correct. If we catch the == case earlier then this makes sense as is. I suppose "if (st->state <= target)" would work too since __cpu_up() already checks. Catching this sooner seems better to me though. > > > ret = lock_device_hotplug_sysfs(); > > if (ret) > > return ret; > > -- > > 2.18.0 > Cheers, Phil --