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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 80C9FCA90AF for ; Tue, 12 May 2020 16:14:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 65C982075E for ; Tue, 12 May 2020 16:14:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728289AbgELQOl (ORCPT ); Tue, 12 May 2020 12:14:41 -0400 Received: from mx2.suse.de ([195.135.220.15]:59296 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728164AbgELQOk (ORCPT ); Tue, 12 May 2020 12:14:40 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 50689ABBD; Tue, 12 May 2020 16:14:42 +0000 (UTC) Date: Tue, 12 May 2020 09:09:15 -0700 From: Davidlohr Bueso To: Oleg Nesterov Cc: akpm@linux-foundation.org, peterz@infradead.org, paulmck@kernel.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: Re: [PATCH 1/2] kernel/sys: only rely on rcu for getpriority(2) Message-ID: <20200512160915.n3plwrwwrlpfqyrs@linux-p48b> References: <20200512000353.23653-1-dave@stgolabs.net> <20200512000353.23653-2-dave@stgolabs.net> <20200512150936.GA28621@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20200512150936.GA28621@redhat.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 12 May 2020, Oleg Nesterov wrote: >On 05/11, Davidlohr Bueso wrote: >> >> Currently the tasklist_lock is shared mainly in order to observe >> the list atomically for the PRIO_PGRP and PRIO_USER cases, as >> the actual lookups are already rcu-safe, > >not really... > >do_each_pid_task(PIDTYPE_PGID) can race with change_pid(PIDTYPE_PGID) >which moves the task from one hlist to another. Yes, it is safe in >that task_struct can't go away. But still this is not right because >do_each_pid_task() can scan the wrong (2nd) hlist. Hmm I didn't think about this case, I guess this is also busted in ioprio_get(2) then. > >> (ii) exit (deletion), this window is small but if a task is >> deleted with the highest nice and it is not observed this would >> cause a change in return semantics. To further reduce the window >> we ignore any tasks that are PF_EXITING in the 'old' version of >> the list. > >can't understand... > >could you explain in details why do you think this PF_EXITING check >makes any sense? My logic was that if the task with the highest prio exited while we were iterating the list, it would not be necessarily seen with rcu and the syscall would return the highest prio of a task that exited; and checking against PF_EXITING was a way to ignore such scenarios as we were going to race with it anyway. At this point it seems that we can just remove the lock for the PRIO_PROCESS case. Thanks, Davidlohr