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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 43A7EC38A24 for ; Thu, 7 May 2020 14:58:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 24D4B20736 for ; Thu, 7 May 2020 14:58:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726951AbgEGO6I (ORCPT ); Thu, 7 May 2020 10:58:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53870 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1725985AbgEGO6I (ORCPT ); Thu, 7 May 2020 10:58:08 -0400 Received: from shards.monkeyblade.net (shards.monkeyblade.net [IPv6:2620:137:e000::1:9]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F1B54C05BD43; Thu, 7 May 2020 07:58:07 -0700 (PDT) Received: from localhost (unknown [IPv6:2601:601:9f00:477::d71]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client did not present a certificate) (Authenticated sender: davem-davemloft) by shards.monkeyblade.net (Postfix) with ESMTPSA id 14AF415C7D92E; Thu, 7 May 2020 07:58:07 -0700 (PDT) Date: Thu, 07 May 2020 07:58:04 -0700 (PDT) Message-Id: <20200507.075804.1424387208635118.davem@davemloft.net> To: mst@redhat.com Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, eric.dumazet@gmail.com, jasowang@redhat.com, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org Subject: Re: [PATCH v2] virtio_net: fix lockdep warning on 32 bit From: David Miller In-Reply-To: <20200507072525.355222-1-mst@redhat.com> References: <20200507072525.355222-1-mst@redhat.com> X-Mailer: Mew version 6.8 on Emacs 26.3 Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.12 (shards.monkeyblade.net [149.20.54.216]); Thu, 07 May 2020 07:58:07 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Michael S. Tsirkin" Date: Thu, 7 May 2020 03:25:56 -0400 > When we fill up a receive VQ, try_fill_recv currently tries to count > kicks using a 64 bit stats counter. Turns out, on a 32 bit kernel that > uses a seqcount. sequence counts are "lock" constructs where you need to > make sure that writers are serialized. > > In turn, this means that we mustn't run two try_fill_recv concurrently. > Which of course we don't. We do run try_fill_recv sometimes from a > softirq napi context, and sometimes from a fully preemptible context, > but the later always runs with napi disabled. > > However, when it comes to the seqcount, lockdep is trying to enforce the > rule that the same lock isn't accessed from preemptible and softirq > context - it doesn't know about napi being enabled/disabled. This causes > a false-positive warning: > > WARNING: inconsistent lock state > ... > inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage. > > As a work around, shut down the warning by switching > to u64_stats_update_begin_irqsave - that works by disabling > interrupts on 32 bit only, is a NOP on 64 bit. > > Reported-by: Thomas Gleixner > Suggested-by: Eric Dumazet > Signed-off-by: Michael S. Tsirkin Applied and queued up for -stable, thanks Michael.