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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 ACEB4C433DF for ; Mon, 25 May 2020 17:45:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8B0C2207DA for ; Mon, 25 May 2020 17:45:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590428745; bh=hqdBoQQ8vJVPjbbwArrndRqys2kpD1XuzZn61430EJI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=z2TY0ngv3WJUiMO9rOQUYMfUn1SmCGYXCb6SEU6qEkTjYOSfJFErEp9PDWZxXN/ch jkrKCE3VraRhfh1oiTSQiL7YawNJDDZ8Acgt0vd1m4cdIMBexeFnthyuMfOFlF7r5/ vrY8yn4fK+vwTIw6qxyNFifb3+h+O8+7D1S4juLE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388975AbgEYRpp (ORCPT ); Mon, 25 May 2020 13:45:45 -0400 Received: from mail.kernel.org ([198.145.29.99]:48582 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388621AbgEYRpo (ORCPT ); Mon, 25 May 2020 13:45:44 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 31420206DD; Mon, 25 May 2020 17:45:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590428744; bh=hqdBoQQ8vJVPjbbwArrndRqys2kpD1XuzZn61430EJI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=MveVTah++a7WlGUvLZ4lmrML8SC0fNODbuYJpeOSVvOyNmezWCiVdZDZ3iLppZkj5 pQt3C+pN75Ww5Hs5RgEqHX7oxdV8oM+r4WXDnqBpUXBRzBtc0D65zwR5EkXk3DPqka R0Qhg20ZVdlmFuYaUZTw6ld2g0hAaTywV+qmRGC0= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 114F03522846; Mon, 25 May 2020 10:45:44 -0700 (PDT) Date: Mon, 25 May 2020 10:45:44 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Andrii Nakryiko , Alan Stern , parri.andrea@gmail.com, will@kernel.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, "andrii.nakryiko@gmail.com" Subject: Re: Some -serious- BPF-related litmus tests Message-ID: <20200525174544.GA21495@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200522003850.GA32698@paulmck-ThinkPad-P72> <20200522094407.GK325280@hirez.programming.kicks-ass.net> <20200522143201.GB32434@rowland.harvard.edu> <20200522174352.GJ2869@paulmck-ThinkPad-P72> <006e2bc6-7516-1584-3d8c-e253211c157e@fb.com> <20200525112521.GD317569@hirez.programming.kicks-ass.net> <20200525154730.GW2869@paulmck-ThinkPad-P72> <20200525170257.GA325280@hirez.programming.kicks-ass.net> <20200525172154.GZ2869@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200525172154.GZ2869@paulmck-ThinkPad-P72> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 25, 2020 at 10:21:54AM -0700, Paul E. McKenney wrote: > On Mon, May 25, 2020 at 07:02:57PM +0200, Peter Zijlstra wrote: > > On Mon, May 25, 2020 at 08:47:30AM -0700, Paul E. McKenney wrote: > > > On Mon, May 25, 2020 at 01:25:21PM +0200, Peter Zijlstra wrote: > > > > > > That is; how can you use a spinlock on the producer side at all? > > > > > > So even trylock is now forbidden in NMI handlers? If so, why? > > > > The litmus tests don't have trylock. > > Fair point. > > > But you made me look at the actual patch: > > > > +static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size) > > +{ > > + unsigned long cons_pos, prod_pos, new_prod_pos, flags; > > + u32 len, pg_off; > > + struct bpf_ringbuf_hdr *hdr; > > + > > + if (unlikely(size > RINGBUF_MAX_RECORD_SZ)) > > + return NULL; > > + > > + len = round_up(size + BPF_RINGBUF_HDR_SZ, 8); > > + cons_pos = smp_load_acquire(&rb->consumer_pos); > > + > > + if (in_nmi()) { > > + if (!spin_trylock_irqsave(&rb->spinlock, flags)) > > + return NULL; > > + } else { > > + spin_lock_irqsave(&rb->spinlock, flags); > > + } > > > > And that is of course utter crap. That's like saying you don't care > > about your NMI data. > > Almost. It is really saying that -if- there is sufficient lock > contention, printk()s will be lost. Just as they always have been if > there is more printk() volume than can be accommodated. s/printk()/BPF output/ One of those days... :-/ Thanx, Paul