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 89CF7C7EE2D for ; Tue, 23 May 2023 15:57:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237494AbjEWP5Y (ORCPT ); Tue, 23 May 2023 11:57:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54820 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237134AbjEWP5V (ORCPT ); Tue, 23 May 2023 11:57:21 -0400 Received: from out02.mta.xmission.com (out02.mta.xmission.com [166.70.13.232]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F31C511A for ; Tue, 23 May 2023 08:57:14 -0700 (PDT) Received: from in02.mta.xmission.com ([166.70.13.52]:43398) by out02.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1q1UNt-005YDB-2I; Tue, 23 May 2023 09:57:13 -0600 Received: from ip68-110-29-46.om.om.cox.net ([68.110.29.46]:48834 helo=email.froward.int.ebiederm.org.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1q1UNr-008Meo-P3; Tue, 23 May 2023 09:57:12 -0600 From: "Eric W. Biederman" To: Oleg Nesterov Cc: Mike Christie , linux@leemhuis.info, nicolas.dichtel@6wind.com, axboe@kernel.dk, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, mst@redhat.com, sgarzare@redhat.com, jasowang@redhat.com, stefanha@redhat.com, brauner@kernel.org References: <20230522025124.5863-1-michael.christie@oracle.com> <20230522025124.5863-4-michael.christie@oracle.com> <20230522123029.GA22159@redhat.com> <20230522174757.GC22159@redhat.com> <20230523121506.GA6562@redhat.com> Date: Tue, 23 May 2023 10:57:04 -0500 In-Reply-To: <20230523121506.GA6562@redhat.com> (Oleg Nesterov's message of "Tue, 23 May 2023 14:15:06 +0200") Message-ID: <87bkib6nxr.fsf@email.froward.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1q1UNr-008Meo-P3;;;mid=<87bkib6nxr.fsf@email.froward.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.110.29.46;;;frm=ebiederm@xmission.com;;;spf=pass X-XM-AID: U2FsdGVkX1/ZYI+rgtjgtOjfAlPhkboPTJIRJCH5jOE= X-SA-Exim-Connect-IP: 68.110.29.46 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Oleg Nesterov writes: > On 05/22, Oleg Nesterov wrote: >> >> Right now I think that "int dead" should die, > > No, probably we shouldn't call get_signal() if we have already > dequeued SIGKILL. Very much agreed. It is one thing to add a patch to move do_exit out of get_signal. It is another to keep calling get_signal after that. Nothing tests that case, and so we get some weird behaviors. >> but let me think tomorrow. > > May be something like this... I don't like it but I can't suggest anything better > right now. > > bool killed = false; > > for (;;) { > ... > > node = llist_del_all(&worker->work_list); > if (!node) { > schedule(); > /* > * When we get a SIGKILL our release function will > * be called. That will stop new IOs from being queued > * and check for outstanding cmd responses. It will then > * call vhost_task_stop to tell us to return and exit. > */ > if (signal_pending(current)) { > struct ksignal ksig; > > if (!killed) > killed = get_signal(&ksig); > > clear_thread_flag(TIF_SIGPENDING); > } > > continue; > } I want to point out that we need to consider not just SIGKILL, but SIGABRT that causes a coredump, as well as the process peforming an ordinary exit(2). All of which will cause get_signal to return SIGKILL in this context. > > ------------------------------------------------------------------------------- > But let me ask a couple of questions. I share most of these questions. > Let's forget this patch, let's look at the > current code: > > node = llist_del_all(&worker->work_list); > if (!node) > schedule(); > > node = llist_reverse_order(node); > ... process works ... > > To me this looks a bit confusing. Shouldn't we do > > if (!node) { > schedule(); > continue; > } > > just to make the code a bit more clear? If node == NULL then > llist_reverse_order() and llist_for_each_entry_safe() will do nothing. > But this is minor. > > > > /* make sure flag is seen after deletion */ > smp_wmb(); > llist_for_each_entry_safe(work, work_next, node, node) { > clear_bit(VHOST_WORK_QUEUED, &work->flags); > > I am not sure about smp_wmb + clear_bit. Once we clear VHOST_WORK_QUEUED, > vhost_work_queue() can add this work again and change work->node->next. > > That is why we use _safe, but we need to ensure that llist_for_each_safe() > completes LOAD(work->node->next) before VHOST_WORK_QUEUED is cleared. > > So it seems that smp_wmb() can't help and should be removed, instead we need > > llist_for_each_entry_safe(...) { > smp_mb__before_atomic(); > clear_bit(VHOST_WORK_QUEUED, &work->flags); > > Also, if the work->fn pointer is not stable, we should read it before > smp_mb__before_atomic() as well. > > No? > > > __set_current_state(TASK_RUNNING); > > Why do we set TASK_RUNNING inside the loop? Does this mean that work->fn() > can return with current->state != RUNNING ? > > > work->fn(work); > > Now the main question. Whatever we do, SIGKILL/SIGSTOP/etc can come right > before we call work->fn(). Is it "safe" to run this callback with > signal_pending() or fatal_signal_pending() ? > > > Finally. I never looked into drivers/vhost/ before so I don't understand > this code at all, but let me ask anyway... Can we change vhost_dev_flush() > to run the pending callbacks rather than wait for vhost_worker() ? > I guess we can't, ->mm won't be correct, but can you confirm? In a conversation long ago I remember hearing that vhost does not support file descriptor passing. Which means all of the file descriptors should be in the same process. Looking at the vhost code what I am seeing happening is that the vhost_worker persists until vhost_dev_cleanup is called from one of the vhost_???_release() functions. The release functions are only called after the last flush function completes. See __fput if you want to trace the details. On one hand this all seems reasonable. On the other hand I am not seeing the code that prevents file descriptor passing. It is probably not the worst thing in the world, but what this means is now if you pass a copy of the vhost file descriptor to another process the vhost_worker will persist, and thus the process will persist until that copy of the file descriptor is closed. Eric