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([2001:ee0:4f4c:210:311c:669b:9c36:4a99]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29dae49b196sm191451135ad.17.2025.12.10.07.33.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Dec 2025 07:33:28 -0800 (PST) Message-ID: Date: Wed, 10 Dec 2025 22:33:17 +0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net] virtio-net: enable all napis before scheduling refill work To: Jason Wang Cc: netdev@vger.kernel.org, "Michael S. Tsirkin" , Xuan Zhuo , =?UTF-8?Q?Eugenio_P=C3=A9rez?= , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, bpf@vger.kernel.org References: <20251208153419.18196-1-minhquangbui99@gmail.com> <66d9f44c-295e-4b62-86ae-a0aff5f062bb@gmail.com> Content-Language: en-US From: Bui Quang Minh In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 12/10/25 12:45, Jason Wang wrote: > On Tue, Dec 9, 2025 at 11:23 PM Bui Quang Minh wrote: >> On 12/9/25 11:30, Jason Wang wrote: >>> On Mon, Dec 8, 2025 at 11:35 PM Bui Quang Minh wrote: >>>> Calling napi_disable() on an already disabled napi can cause the >>>> deadlock. In commit 4bc12818b363 ("virtio-net: disable delayed refill >>>> when pausing rx"), to avoid the deadlock, when pausing the RX in >>>> virtnet_rx_pause[_all](), we disable and cancel the delayed refill work. >>>> However, in the virtnet_rx_resume_all(), we enable the delayed refill >>>> work too early before enabling all the receive queue napis. >>>> >>>> The deadlock can be reproduced by running >>>> selftests/drivers/net/hw/xsk_reconfig.py with multiqueue virtio-net >>>> device and inserting a cond_resched() inside the for loop in >>>> virtnet_rx_resume_all() to increase the success rate. Because the worker >>>> processing the delayed refilled work runs on the same CPU as >>>> virtnet_rx_resume_all(), a reschedule is needed to cause the deadlock. >>>> In real scenario, the contention on netdev_lock can cause the >>>> reschedule. >>>> >>>> This fixes the deadlock by ensuring all receive queue's napis are >>>> enabled before we enable the delayed refill work in >>>> virtnet_rx_resume_all() and virtnet_open(). >>>> >>>> Fixes: 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx") >>>> Reported-by: Paolo Abeni >>>> Closes: https://netdev-ctrl.bots.linux.dev/logs/vmksft/drv-hw-dbg/results/400961/3-xdp-py/stderr >>>> Signed-off-by: Bui Quang Minh >>>> --- >>>> drivers/net/virtio_net.c | 59 +++++++++++++++++++--------------------- >>>> 1 file changed, 28 insertions(+), 31 deletions(-) >>>> >>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c >>>> index 8e04adb57f52..f2b1ea65767d 100644 >>>> --- a/drivers/net/virtio_net.c >>>> +++ b/drivers/net/virtio_net.c >>>> @@ -2858,6 +2858,20 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq, >>>> return err != -ENOMEM; >>>> } >>>> >>>> +static void virtnet_rx_refill_all(struct virtnet_info *vi) >>>> +{ >>>> + bool schedule_refill = false; >>>> + int i; >>>> + >>>> + enable_delayed_refill(vi); >>> This seems to be still racy? >>> >>> For example, in virtnet_open() we had: >>> >>> static int virtnet_open(struct net_device *dev) >>> { >>> struct virtnet_info *vi = netdev_priv(dev); >>> int i, err; >>> >>> for (i = 0; i < vi->max_queue_pairs; i++) { >>> err = virtnet_enable_queue_pair(vi, i); >>> if (err < 0) >>> goto err_enable_qp; >>> } >>> >>> virtnet_rx_refill_all(vi); >>> >>> So NAPI and refill work is enabled in this case, so the refill work >>> could be scheduled and run at the same time? >> Yes, that's what we expect. We must ensure that refill work is scheduled >> only when all NAPIs are enabled. The deadlock happens when refill work >> is scheduled but there are still disabled RX NAPIs. > Just to make sure we are on the same page, I meant, after refill work > is enabled, rq0 is NAPI is enabled, in this case the refill work could > be triggered by the rq0's NAPI so we may end up in the refill work > that it tries to disable rq1's NAPI while holding the netdev lock. I don't quite get your point. The current deadlock scenario is this virtnet_rx_resume_all napi_enable(rq0) (the rq1 napi is still disabled) enable_refill_work refill_work napi_disable(rq0) -> still okay napi_enable(rq0) -> still okay napi_disable(rq1) -> hold netdev_lock     -> stuck inside the while loop in napi_disable_locked             while (val & (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC)) {                 usleep_range(20, 200);                 val = READ_ONCE(n->state);             } napi_enable(rq1) -> stuck while trying to acquire the netdev_lock The problem is that we must not call napi_disable() on an already disabled NAPI (rq1's NAPI in the example). In the new virtnet_open static int virtnet_open(struct net_device *dev) { struct virtnet_info *vi = netdev_priv(dev); int i, err; // Note that at this point, refill work is still disabled, vi->refill_enabled == false, // so even if virtnet_receive is called, the refill_work will not be scheduled. for (i = 0; i < vi->max_queue_pairs; i++) { err = virtnet_enable_queue_pair(vi, i); if (err < 0) goto err_enable_qp; } // Here all RX NAPIs are enabled so it's safe to enable refill work again virtnet_rx_refill_all(vi); Thanks, Quang Minh.