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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 C8657C432C0 for ; Wed, 27 Nov 2019 09:35:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A09B620656 for ; Wed, 27 Nov 2019 09:35:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726320AbfK0Jf2 (ORCPT ); Wed, 27 Nov 2019 04:35:28 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:44127 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726130AbfK0Jf2 (ORCPT ); Wed, 27 Nov 2019 04:35:28 -0500 Received: from bigeasy by Galois.linutronix.de with local (Exim 4.80) (envelope-from ) id 1iZtjF-0003Tf-UB; Wed, 27 Nov 2019 10:35:21 +0100 Date: Wed, 27 Nov 2019 10:35:21 +0100 From: Sebastian Andrzej Siewior To: Eric Dumazet Cc: netdev , "David S. Miller" , Thomas Gleixner Subject: Re: [PATCH net 2/2] net: gro: Let the timeout timer expire in softirq context with `threadirqs' Message-ID: <20191127093521.6achiubslhv7u46c@linutronix.de> References: <20191126222013.1904785-1-bigeasy@linutronix.de> <20191126222013.1904785-3-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2019-11-26 14:39:47 [-0800], Eric Dumazet wrote: > On Tue, Nov 26, 2019 at 2:20 PM Sebastian Andrzej Siewior > wrote: > > > > The timer callback (napi_watchdog()) invokes __napi_schedule_irqoff() > > with disabled interrupts. With the `threadirqs' commandline option all > > interrupt handler are threaded and using __napi_schedule_irqoff() is not > > an issue because everyone is using it in threaded context which is > > synchronised with local_bh_disable(). > > The napi_watchdog() timer is still expiring in hardirq context and may > > interrupt a threaded handler which is in the middle of > > __napi_schedule_irqoff() leading to list corruption. > > Sorry, I do not understand this changelog. > > Where/how do you get list corruption exactly ? > > __napi_schedule_irqoff() _has_ to be called with hard IRQ disabled. > > Please post a stack trace. I don't have a stack trace, this is based on review. __napi_schedule_irqoff() is used in IRQ context and this is only the primary IRQ handler. There is no other in-IRQ usage (like SMP-function call or so (excluding the HV network driver)) but there is the hrtimer. With `threadirqs' you don't have the in-IRQ usage from the IRQ handler anymore. The IRQ-handler for two different NICs don't interrupt/ preempt each other because the handler is invoked with disabled softirq (which also disables preemption). This is all safe. The hrtimer fires always in IRQ context no matter if `threadirqs' is used or not. Which brings us to the following race condition: One CPU, 2 NICs: threaded_IRQ of NIC1 hard-irq context, hrtimer local_bh_disable() nic_irq_handler() if (napi_schedule_prep()) __napi_schedule_irqoff() ____napi_schedule(this_cpu_ptr(&softnet_data), n); -> list_add_tail(, &sd->poll_list); hrtimer_interrupt() __hrtimer_run_queues() napi_watchdog() __napi_schedule_irqoff() ____napi_schedule(this_cpu_ptr(&softnet_data), n); -> list_add_tail(&napi->poll_list, &sd->poll_list); The same per-CPU list modified again. If the callback is moved to softirq instead: threaded_IRQ of NIC1 hard-irq context, hrtimer local_bh_disable() nic_irq_handler() if (napi_schedule_prep()) __napi_schedule_irqoff() ____napi_schedule(this_cpu_ptr(&softnet_data), n); -> list_add_tail(, &sd->poll_list); hrtimer_interrupt() raise_softirq_irqoff(HRTIMER_SOFTIRQ); list_add_tail() completes local_bh_enable() if (unlikely(!in_interrupt() && local_softirq_pending())) { do_softirq(); napi_watchdog() __napi_schedule_irqoff() ____napi_schedule(this_cpu_ptr(&softnet_data), n); list_add_tail(, &sd->poll_list); > > Let the napi_watchdog() expire in softirq context if `threadirqs' is > > used. > > > > Fixes: 3b47d30396bae ("net: gro: add a per device gro flush timer") > > Are you sure this commit is the root cause of the problem you see ? This commit was introduced in v3.19-rc1 and threadirqs was introduced in commit 8d32a307e4faa ("genirq: Provide forced interrupt threading") which is v2.6.39-rc1. Based on my explanation above this problem exists with the timer and `threadirqs'. The hrtimer always expired in hardirq context. > > Cc: Eric Dumazet > > Signed-off-by: Sebastian Andrzej Siewior > > --- > > net/core/dev.c | 6 +++++- > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > diff --git a/net/core/dev.c b/net/core/dev.c > > index 99ac84ff398f4..ec533d20931bc 100644 > > --- a/net/core/dev.c > > +++ b/net/core/dev.c > > @@ -5994,6 +5994,8 @@ bool napi_complete_done(struct napi_struct *n, int work_done) > > napi_gro_flush(n, !!timeout); > > if (timeout) > > hrtimer_start(&n->timer, ns_to_ktime(timeout), > > + force_irqthreads ? > > Honestly something is wrong with this patch, force_irqthreads should > not be used in net/ territory, > that is some layering problem. I'm not aware of an other problems of this kind. Most drivers do spin_lock_irqsave() and in that case in does not matter if the interrupt is threaded or not vs the hrtimer callback. Which means they do not assume that the IRQ handler runs with interrupts disabled. The timeout timers are usually timer_list timer which run in softirq context and since the force-IRQ-thread runs also with disabled softirq it is fine using just spin_lock(). If you don't want this here maybe tglx can add some hrtimer annotation. Sebastian