From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: Some -serious- BPF-related litmus tests Date: Mon, 25 May 2020 10:21:54 -0700 Message-ID: <20200525172154.GZ2869@paulmck-ThinkPad-P72> 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> Reply-To: paulmck@kernel.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail.kernel.org ([198.145.29.99]:44786 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388230AbgEYRVz (ORCPT ); Mon, 25 May 2020 13:21:55 -0400 Content-Disposition: inline In-Reply-To: <20200525170257.GA325280@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: 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" 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. Thanx, Paul