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 13ED4C433EF for ; Fri, 27 May 2022 13:22:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1351697AbiE0NWs (ORCPT ); Fri, 27 May 2022 09:22:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36744 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242354AbiE0NWr (ORCPT ); Fri, 27 May 2022 09:22:47 -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 3C6E43B54D for ; Fri, 27 May 2022 06:22:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1653657765; 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=XQbqWL32WgNp+vr6+IdXD3tIFXkuGIZQKgAX0wF6o5I=; b=gcvQQEvCBfc2kCXZjD6QzbN9Z+P5PK7yGBePhAEZkGPkIq6uoqG8O8RuLwyp3FQl1uvE72 t8kYAGp82JdcTIEMOxJ/vDfleeY1F9SfKesa7RB/Ys6A80vaR6DA8LEpN/bhzoR26sr7bn dZcbhVkW2nsz3NTRTeGT1FkW5Nuagsk= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-60-DwDo9MTHMZKK_lUYFY3QQg-1; Fri, 27 May 2022 09:22:40 -0400 X-MC-Unique: DwDo9MTHMZKK_lUYFY3QQg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id B5A343C14103; Fri, 27 May 2022 13:22:39 +0000 (UTC) Received: from pauld.bos.csb (dhcp-17-51.bos.redhat.com [10.18.17.51]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 93DC31121315; Fri, 27 May 2022 13:22:39 +0000 (UTC) Date: Fri, 27 May 2022 09:22:38 -0400 From: Phil Auld To: Valentin Schneider Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra Subject: Re: [PATCH v2 1/2] cpuhp: make target_store() a nop when target == state Message-ID: <20220527132156.GB26124@pauld.bos.csb> References: <20220526160615.7976-1-pauld@redhat.com> <20220526160615.7976-2-pauld@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 27, 2022 at 10:38:24AM +0100 Valentin Schneider wrote: > On 26/05/22 12:06, Phil Auld wrote: > > writing the current state back in 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); > > > > Fix this by letting state==target fall through in the target_store() > > conditional. > > > > To go back on my data race paranoia: writes to both cpu$x/online and > cpu$x/hotplug/target are serialized by device_hotplug_lock, and so are the > exported kernel hotplug functions ({add, remove}_cpu()). > > That's not cpu_add_remove_lock as I was looking for, but that's still all > under one lock, so I think we're good. Sorry for that! > Right. This catches it up higher so that we don't get into the code that starts actually changing things. I wonder now in the state == target case if we should make sure st->target == target. With the second patch it's less likely to be needed. Thoughts? Maybe I'll include that if/when I have code to keep cpux/online in sync with st->state and cpu_online_mask. > > Signed-off-by: Phil Auld > > Reviewed-by: Valentin Schneider Thanks! Cheers, Phil > > > --- > > kernel/cpu.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/cpu.c b/kernel/cpu.c > > index d0a9aa0b42e8..cdb6ac10ad94 100644 > > --- a/kernel/cpu.c > > +++ b/kernel/cpu.c > > @@ -2315,7 +2315,7 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr, > > > > if (st->state < target) > > ret = cpu_up(dev->id, target); > > - else > > + else if (st->state > target) > > ret = cpu_down(dev->id, target); > > out: > > unlock_device_hotplug(); > > -- > > 2.18.0 > --