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[109.66.7.121]) by smtp.gmail.com with ESMTPSA id 185sm491014wmc.32.2020.05.05.17.43.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 05 May 2020 17:43:18 -0700 (PDT) Date: Tue, 5 May 2020 20:43:16 -0400 From: "Michael S. Tsirkin" To: Eric Dumazet Cc: Thomas Gleixner , netdev@vger.kernel.org, Jason Wang , Peter Zijlstra Subject: Re: [BUG] Inconsistent lock state in virtnet poll Message-ID: <20200505204015-mutt-send-email-mst@kernel.org> References: <87lfm6oa7b.fsf@nanos.tec.linutronix.de> <20200505120352-mutt-send-email-mst@kernel.org> <87v9lanher.fsf@nanos.tec.linutronix.de> <98c4d934-5a27-1cf7-119a-ce0c5a501864@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <98c4d934-5a27-1cf7-119a-ce0c5a501864@gmail.com> Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Tue, May 05, 2020 at 03:40:09PM -0700, Eric Dumazet wrote: > > > On 5/5/20 3:30 PM, Thomas Gleixner wrote: > > "Michael S. Tsirkin" writes: > >> On Tue, May 05, 2020 at 02:08:56PM +0200, Thomas Gleixner wrote: > >>> > >>> The following lockdep splat happens reproducibly on 5.7-rc4 > >> > >>> ================================ > >>> WARNING: inconsistent lock state > >>> 5.7.0-rc4+ #79 Not tainted > >>> -------------------------------- > >>> inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage. > >>> ip/356 [HC0[0]:SC1[1]:HE1:SE0] takes: > >>> f3ee4cd8 (&syncp->seq#2){+.?.}-{0:0}, at: net_rx_action+0xfb/0x390 > >>> {SOFTIRQ-ON-W} state was registered at: > >>> lock_acquire+0x82/0x300 > >>> try_fill_recv+0x39f/0x590 > >> > >> Weird. Where does try_fill_recv acquire any locks? > > > > u64_stats_update_begin(&rq->stats.syncp); > > > > That's a 32bit kernel which uses a seqcount for this. sequence counts > > are "lock" constructs where you need to make sure that writers are > > serialized. > > > > Actually the problem at hand is that try_fill_recv() is called from > > fully preemptible context initialy and then from softirq context. > > > > Obviously that's for the open() path a non issue, but lockdep does not > > know about that. OTOH, there is other code which calls that from > > non-softirq context. > > > > The hack below made it shut up. It's obvioulsy not ideal, but at least > > it let me look at the actual problem I was chasing down :) > > > > Thanks, > > > > tglx > > > > 8<----------- > > --- a/drivers/net/virtio_net.c > > +++ b/drivers/net/virtio_net.c > > @@ -1243,9 +1243,11 @@ static bool try_fill_recv(struct virtnet > > break; > > } while (rq->vq->num_free); > > if (virtqueue_kick_prepare(rq->vq) && virtqueue_notify(rq->vq)) { > > + local_bh_disable(); > > Or use u64_stats_update_begin_irqsave() whic is a NOP on 64bit kernels I applied this, but am still trying to think of something that is 0 overhead for all configs. Maybe we can select a lockdep class depending on whether napi is enabled? > > u64_stats_update_begin(&rq->stats.syncp); > > rq->stats.kicks++; > > u64_stats_update_end(&rq->stats.syncp); > > + local_bh_enable(); > > } > > > > return !oom; > >