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 0AEE8C7EE23 for ; Tue, 23 May 2023 12:16:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235469AbjEWMQX (ORCPT ); Tue, 23 May 2023 08:16:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45958 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236760AbjEWMQV (ORCPT ); Tue, 23 May 2023 08:16:21 -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 ESMTPS id 706D7109 for ; Tue, 23 May 2023 05:15:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684844130; 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=RMfXzJcNmXtD5s5OdLp9quleEGVvmdXTdhN2MxgG29w=; b=bnFjOgny1jFxIB0eL8FZIFM2sub7mevouwziBdjb2EGo8IbSkWa19GhYjT9zruI+hehZvG fzMGPvSfvkMuXRdRfZFULO7zAeNa7R2ctcmKGYRGjL6jYY8JB2BDYKEU2IQW0CRdlTkH3d VcSr1WWLBn5RD96PVSGWcs1/KHrLf9c= 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-140-APhMgIePNlaesPXpS6PWkg-1; Tue, 23 May 2023 08:15:27 -0400 X-MC-Unique: APhMgIePNlaesPXpS6PWkg-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 D46B9811E7F; Tue, 23 May 2023 12:15:26 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.45.226.170]) by smtp.corp.redhat.com (Postfix) with SMTP id 30AA9140E95D; Tue, 23 May 2023 12:15:23 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Tue, 23 May 2023 14:15:10 +0200 (CEST) Date: Tue, 23 May 2023 14:15:06 +0200 From: Oleg Nesterov To: Mike Christie Cc: linux@leemhuis.info, nicolas.dichtel@6wind.com, axboe@kernel.dk, ebiederm@xmission.com, 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 Subject: Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression Message-ID: <20230523121506.GA6562@redhat.com> References: <20230522025124.5863-1-michael.christie@oracle.com> <20230522025124.5863-4-michael.christie@oracle.com> <20230522123029.GA22159@redhat.com> <20230522174757.GC22159@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230522174757.GC22159@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > 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; } ------------------------------------------------------------------------------- But let me ask a couple of 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? Oleg.