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 20E39CA90AF for ; Tue, 12 May 2020 17:03:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 03DB6206D3 for ; Tue, 12 May 2020 17:03:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728163AbgELRDu (ORCPT ); Tue, 12 May 2020 13:03:50 -0400 Received: from mx2.suse.de ([195.135.220.15]:48286 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726031AbgELRDt (ORCPT ); Tue, 12 May 2020 13:03:49 -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 B0C0BAC4D; Tue, 12 May 2020 17:03:50 +0000 (UTC) Date: Tue, 12 May 2020 09:58:24 -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: <20200512165824.t6ktwllqlvkiingv@linux-p48b> References: <20200512000353.23653-1-dave@stgolabs.net> <20200512000353.23653-2-dave@stgolabs.net> <20200512150936.GA28621@redhat.com> <20200512160915.n3plwrwwrlpfqyrs@linux-p48b> <20200512164130.GC28621@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20200512164130.GC28621@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 05/12, Davidlohr Bueso wrote: >> >> On Tue, 12 May 2020, Oleg Nesterov wrote: >> >> >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. > >agreed... > >> > >> >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. > >Sorry, still can't understand. The PF_EXITING flag is not protected by >tasklist_lock or rcu_lock. Sorry for not making my idea clear, perhaps it's complete garbage. Right, but setting the flag is an indication that the tasklist_lock will be taken and removed from the list, and therefore we could optimistically avoid considering that task altogether instead of relying on the old copy of the list. It's not perfect, but it does reduce the window in which getpriority() can return a stale value(?). At least this is how I justify it. Otoh this also opens a window in where the lockless version can ignore highest prio task when the locked version would otherwise consider it. So it might not be worth it. > >OK, if nothing else. Suppose that a prgp has a single process P, this >proces has already exited but its parent didn't do wait(). > >Currently getpriority() returns task_nice(P). With the PF_EXITING check >it will return -ESRCH. Hmm? Yes, that would need fixing but you don't seem to be buying the idea in the first place. Thanks, Davidlohr