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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 37C30C28CF6 for ; Tue, 24 Jul 2018 23:43:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D8A8C2064D for ; Tue, 24 Jul 2018 23:43:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D8A8C2064D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728006AbeGYAw3 (ORCPT ); Tue, 24 Jul 2018 20:52:29 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:41614 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727353AbeGYAw3 (ORCPT ); Tue, 24 Jul 2018 20:52:29 -0400 Received: from akpm3.svl.corp.google.com (unknown [104.133.9.92]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id B6EBAAF8; Tue, 24 Jul 2018 23:43:35 +0000 (UTC) Date: Tue, 24 Jul 2018 16:43:34 -0700 From: Andrew Morton To: Davidlohr Bueso Cc: Peter Zijlstra , jbaron@akamai.com, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org Subject: Re: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible Message-Id: <20180724164334.718ab5715b5d5ad0c6c005dd@linux-foundation.org> In-Reply-To: <20180721183127.3busfa335zlcjeox@linux-r8p5> References: <20180720172956.2883-1-dave@stgolabs.net> <20180720124212.7260d76d83e2b8e5e3349ea5@linux-foundation.org> <20180720200559.27nc7j2rrxpy5p3n@linux-r8p5> <20180720134429.1ba61018934b084bb2e17bdb@linux-foundation.org> <20180721172120.kbdu4euc2wn4xzgf@linux-r8p5> <20180721173944.GV2494@hirez.programming.kicks-ass.net> <20180721183127.3busfa335zlcjeox@linux-r8p5> X-Mailer: Sylpheed 3.6.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 21 Jul 2018 11:31:27 -0700 Davidlohr Bueso wrote: > On Sat, 21 Jul 2018, Peter Zijlstra wrote: > > >On Sat, Jul 21, 2018 at 10:21:20AM -0700, Davidlohr Bueso wrote: > >> On Fri, 20 Jul 2018, Andrew Morton wrote: > >> > >> > We could open-code it locally. Add a couple of > >> > WARN_ON_ONCE(irqs_disabled())? That might need re-benchmarking with > >> > Xen but surely just reading the thing isn't too expensive? > >> > >> We could also pass on the responsibility to lockdep and just use > >> lockdep_assert_irqs_disabled(). But I guess that would be less effective > >> than to just open code it in epoll without lockdep -- note that over 80 > >> places in the kernel do this. > > > >The lockdep thing is relatively recent. I think someone proposed to go > >replace a bunch of the open-coded ones at some point. > > For the open coded checks, I'm seeing a small (1-2% ish) cost for bare > metal on workload 1). I don't see (via code inspection) any additional > overhead in xen either. While negligible in the overall of things, I do > like the idea of lockdep handling it nonetheless. It's good that the overhead goes away when lockdep is disabled. That's a big advantage over open-coding it. > I can add the open coded version if people really feel that it would catch > more bugs (no lockdep users out there in production afaik :) in the long > term; but if lockdep is where things are headed... > > ... > > --- a/fs/eventpoll.c > +++ b/fs/eventpoll.c > @@ -670,6 +670,9 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep, > struct epitem *epi, *nepi; > LIST_HEAD(txlist); > > + /* must not be called with irqs off */ > + lockdep_assert_irqs_enabled(); Although I be the humble comment's most avid fan, these ones are too obvious to be worth the space ;) Would you survive if I zapped 'em? --- a/fs/eventpoll.c~fs-epoll-robustify-irq-safety-with-lockdep_assert_irqs_enabled-fix +++ a/fs/eventpoll.c @@ -670,7 +670,6 @@ static __poll_t ep_scan_ready_list(struc struct epitem *epi, *nepi; LIST_HEAD(txlist); - /* must not be called with irqs off */ lockdep_assert_irqs_enabled(); /* @@ -767,7 +766,6 @@ static int ep_remove(struct eventpoll *e { struct file *file = epi->ffd.file; - /* must not be called with irqs off */ lockdep_assert_irqs_enabled(); /* @@ -1418,7 +1416,6 @@ static int ep_insert(struct eventpoll *e struct epitem *epi; struct ep_pqueue epq; - /* must not be called with irqs off */ lockdep_assert_irqs_enabled(); user_watches = atomic_long_read(&ep->user->epoll_watches); @@ -1549,7 +1546,6 @@ static int ep_modify(struct eventpoll *e int pwake = 0; poll_table pt; - /* must not be called with irqs off */ lockdep_assert_irqs_enabled(); init_poll_funcptr(&pt, NULL); _