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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 8D66BC433DB for ; Fri, 8 Jan 2021 09:31:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4072C23372 for ; Fri, 8 Jan 2021 09:31:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728147AbhAHJb2 convert rfc822-to-8bit (ORCPT ); Fri, 8 Jan 2021 04:31:28 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:55568 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727629AbhAHJbY (ORCPT ); Fri, 8 Jan 2021 04:31:24 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-146-1FI48KQOPHOmmMChUE2yHA-1; Fri, 08 Jan 2021 09:29:43 +0000 X-MC-Unique: 1FI48KQOPHOmmMChUE2yHA-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Fri, 8 Jan 2021 09:29:42 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Fri, 8 Jan 2021 09:29:42 +0000 From: David Laight To: 'Al Viro' , Linus Torvalds CC: kernel test robot , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Peter Zijlstra , LKML , "lkp@lists.01.org" , kernel test robot , "Huang, Ying" , Feng Tang , "zhengjun.xing@intel.com" Subject: RE: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression Thread-Topic: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression Thread-Index: AQHW5S+2ndv+N/v8PE2rPRBCL8mR5Koddc2w Date: Fri, 8 Jan 2021 09:29:42 +0000 Message-ID: References: <20210107134723.GA28532@xsang-OptiPlex-9020> <20210107183358.GG3579531@ZenIV.linux.org.uk> <20210107190445.GK3579531@ZenIV.linux.org.uk> <20210107195734.GL3579531@ZenIV.linux.org.uk> In-Reply-To: <20210107195734.GL3579531@ZenIV.linux.org.uk> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Al Viro > Sent: 07 January 2021 19:58 > > On Thu, Jan 07, 2021 at 11:33:36AM -0800, Linus Torvalds wrote: > > > In fact, even some threaded app that does what I suspect it could do > > would likely be ok with it 99% of the time. Because the situation > > where you change the fd in the poll array is likely not the common > > case, and even if some -1 file descriptor gets overwritten by a valid > > one by the poll() system call again, it probably ends up being very > > hard to see a failure. > > > > Which just makes me even more nervous. > > Hmm... But anything like that will have another problem - we do > copyin only once. And we repeat fdget() on each iteration of > do_poll() loop. Sure, we don't actually put anything on the > queues after the first time around, and __pollwait() keeps the > ones we are actually waiting for pinned, but... If another > thread stores -1 to ->fd, then closes what used to be there > and moves on, what will it see? ->poll() calls will be done > for whatever file we'd reused the descriptor for. Sure, > the kernel won't break, but the caller of poll() would need > to be very careful about what it sees... > > Frankly, I'd consider seeing that kind of games in the userland > as a big red flag; I'm not saying it's OK to break the suckers > even worse than they are now, but I'm curious whether anything > in the userland does it *and* how many bugs does it have around > those uses of poll()... It is much more likely that an application will change the 'events' field - in particular enabling POLLOUT if a write() returned EAGAIN. It could also change the fd, but defer doing the actual close() until much later - that needs to be synchronised between the application threads. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)