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[80.230.24.117]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47f4634e029sm8851487f8f.3.2026.07.14.06.17.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Jul 2026 06:17:43 -0700 (PDT) Date: Tue, 14 Jul 2026 09:17:40 -0400 From: "Michael S. Tsirkin" To: Jinqian Yang Cc: jasowang@redhat.com, xuanzhuo@linux.alibaba.com, eperezma@redhat.com, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, netdev@vger.kernel.org, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, liuyonglong@huawei.com, wangzhou1@hisilicon.com, linuxarm@huawei.com Subject: Re: [PATCH] virtio_net: fix infinite loop in virtnet_poll_cleantx when device is broken Message-ID: <20260714091622-mutt-send-email-mst@kernel.org> References: <20260713132025.703147-1-yangjinqian1@huawei.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20260713132025.703147-1-yangjinqian1@huawei.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: NT19voxJtdia-znE7jAClhrVXirImbor72HtGO1xxyo_1784035065 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Mon, Jul 13, 2026 at 09:20:25PM +0800, Jinqian Yang wrote: > virtnet_poll_cleantx() contains a do-while loop that cleans up > transmitted TX buffers and calls virtqueue_enable_cb_delayed() to check > whether more buffers need processing. When the virtio backend stops > responding during guest reboot, used->idx is never updated, so > virtqueue_enable_cb_delayed() always returns false and the loop never > terminates. Then it will block reboot process, and the guest will hang. > > The problem occurs during guest reboot under network traffic: > > 1. kernel_restart() -> device_shutdown() traverses the device list > 2. virtio_dev_shutdown() calls virtio_break_device() which sets > vq->broken = true > 3. virtio_dev_shutdown() then calls virtio_synchronize_cbs() to wait > for in-flight callbacks to complete > 4. A virtio interrupt fires, softirq is deferred to ksoftirqd which > calls net_rx_action() -> virtnet_poll() -> virtnet_poll_cleantx() > 5. virtnet_poll_cleantx() enters the do-while loop and never exits > because the QEMU backend has stopped updating used->idx, despite > vq->broken having been set to true in step 2. > > Since the loop runs inside ksoftirqd (a SCHED_OTHER kthread), it is > visible to the scheduler and does not trigger a hard lockup. However, > the kthread never leaves the loop, so RCU detects it as a CPU stall > and reports it periodically. Meanwhile, the reboot process remains > blocked in device_shutdown() because virtio_dev_shutdown() cannot > complete its synchronization step, and the guest hangs permanently. > > This can be reproduced on a guest with a virtio-net device: run iperf3 > traffic in the guest, then trigger reboot. The reboot occasionally hangs > permanently with RCU stall on ksoftirqd. > > Observed on ARM64 KVM guest: > > CPU#1 RCU stall (ksoftirqd/1), repeated periodically: > virtqueue_enable_cb_delayed_split <- virtnet_poll <- __napi_poll <- > net_rx_action <- handle_softirqs <- run_ksoftirqd <- > smpboot_thread_fn <- kthread > > Fix by adding a virtqueue_is_broken() check to the loop condition, so > that the loop exits immediately when the device is broken, allowing > the device shutdown to proceed. > > Signed-off-by: Jinqian Yang I'd expect lots of drivers have this issue? Wouldn't it make more sense to check virtqueue_is_broken in virtqueue_enable_cb_delayed/virtqueue_enable_cb? This way it works for all drivers. > --- > drivers/net/virtio_net.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > index 7d2eeb9b1226..c8d2d420c31d 100644 > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -2970,7 +2970,8 @@ static void virtnet_poll_cleantx(struct receive_queue *rq, int budget) > do { > virtqueue_disable_cb(sq->vq); > free_old_xmit(sq, txq, !!budget); > - } while (unlikely(!virtqueue_enable_cb_delayed(sq->vq))); > + } while (!virtqueue_is_broken(sq->vq) && > + unlikely(!virtqueue_enable_cb_delayed(sq->vq))); > > if (sq->vq->num_free >= MAX_SKB_FRAGS + 2) > virtnet_tx_wake_queue(vi, sq); > -- > 2.33.0