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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 E26F7C10F05 for ; Wed, 20 Mar 2019 17:30:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C034C21841 for ; Wed, 20 Mar 2019 17:30:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727625AbfCTRar (ORCPT ); Wed, 20 Mar 2019 13:30:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53996 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727503AbfCTRaq (ORCPT ); Wed, 20 Mar 2019 13:30:46 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id CA6BE4627A; Wed, 20 Mar 2019 17:30:45 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.34]) by smtp.corp.redhat.com (Postfix) with SMTP id 7D4F419C59; Wed, 20 Mar 2019 17:30:43 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Wed, 20 Mar 2019 18:30:44 +0100 (CET) Date: Wed, 20 Mar 2019 18:30:42 +0100 From: Oleg Nesterov To: Michal Hocko Cc: syzbot , chanho.min@lge.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, pavel@ucw.cz, rafael.j.wysocki@intel.com, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk, Ingo Molnar Subject: Re: WARNING: syz-executor still has locks held! Message-ID: <20190320173041.GE21673@redhat.com> References: <0000000000000e4a41057b6c9792@google.com> <0000000000004cdec6058485b2ce@google.com> <20190320131655.GA8696@dhcp22.suse.cz> <20190320132410.GB21673@redhat.com> <20190320132936.GB8696@dhcp22.suse.cz> <20190320150053.GD21673@redhat.com> <20190320151242.GC8696@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190320151242.GC8696@dhcp22.suse.cz> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Wed, 20 Mar 2019 17:30:46 +0000 (UTC) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 03/20, Michal Hocko wrote: > > [Cc Ingo and Chanho Min - the thread starts here > http://lkml.kernel.org/r/0000000000004cdec6058485b2ce@google.com] > > On Wed 20-03-19 16:00:54, Oleg Nesterov wrote: > > On 03/20, Michal Hocko wrote: > > > > > > On Wed 20-03-19 14:24:11, Oleg Nesterov wrote: > > > > On 03/20, Michal Hocko wrote: > > > > > > > > > > Yes we do hold the cgred mutex while calling freezable_schedule but why > > > > > are we getting a warning is not really clear to me. The task should be > > > > > hidden from the freezer so why do we warn at all? > > > > > > > > try_to_freeze() calls debug_check_no_locks_held() and this makes sense. > > > > > > Yes it does. But it already ignores PF_NOFREEZE tasks and I fail to see > > > why is PF_FREEZER_SKIP any different. > > > > But they differ. PF_NOFREEZE is a "sticky" flag for kthreads. Set by default, > > cleared by set_freezable() if you want a freezable kthread. > > > > PF_FREEZER_SKIP means that a sleeping freezable task will call try_to_freeze() > > right after schedule() returns, so try_to_freeze_tasks() can safely count it as > > "already frozen". > > But the fundamental semantic is the same right? Both might be sitting on > locks that might interfere with other tasks and we should be _extra_ > careful when using them. In an ideal world, none of them is really > needed. Ah, it seems that we misunderstood each other... see below. > So my question remains. Can we drop the warning for PF_FREEZER_SKIP > tasks as well? But why? It is obviously wrong to call try_to_freeze() with a lock held. Probably you meant the if (!(current->flags & PF_NOFREEZE)) check in try_to_freeze() when you said "already ignores PF_NOFREEZE tasks". I am not sure we actually need this check, a PF_NOFREEZE kthread shouldn't call try_to_freeze() at least directly. However, note that freezing() will return false if PF_NOFREEZE is set, so try_to_freeze() is nop in this case. Probably this is why PF_NOFREEZE is also checked before debug_check_no_locks_held(). > > > as removing the cgred is way way too complicated. > > > > We need to do this anyway, this leads to other more serious problems... > > Yes but this is far away and it doesn't really seem like a stable tree > material strace -f can hang ;) so this is the stable material. Oleg.