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[2a02:3100:b3ca:3a01:3deb:81e4:dc4:fbe8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a03feed16bsm11717875e9.4.2026.10.01.16.30.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 01 Oct 2026 16:30:47 -0700 (PDT) Date: Fri, 2 Oct 2026 01:30:45 +0200 From: Karl Mehltretter To: Sebastian Andrzej Siewior Cc: netdev@vger.kernel.org, "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Clark Williams , Steven Rostedt , Stephen Hemminger , Arend van Spriel , linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, stable@vger.kernel.org Subject: Re: [PATCH net v2 1/2] netpoll: use a raw lock for the deferred transmit queue Message-ID: References: <20260928064239.32456-1-kmehltretter@gmail.com> <20260930192103.62973-2-kmehltretter@gmail.com> <20261001074819.gQ3w8q6P@linutronix.de> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261001074819.gQ3w8q6P@linutronix.de> On Thu, Oct 01, 2026 at 09:48:19AM +0100, Sebastian Andrzej Siewior wrote: > On 2026-09-30 21:21:02 [+0200], Karl Mehltretter wrote: > > netpoll_send_skb() calls __netpoll_send_skb() with hard interrupts > > disabled. When direct transmission cannot complete, the latter queues > > the skb with skb_queue_tail(). The sk_buff_head lock may sleep on > > PREEMPT_RT: > > > > BUG: sleeping function called from invalid context > > in_atomic(): 0, irqs_disabled(): 1, non_block: 0 > > rt_spin_lock > > skb_queue_tail > > netpoll_send_skb > > How is this possible? netpoll is only used by netconsole right? And this > is CON_NBCON so it only prints threaded. What is the missing piece? > The call is from the NBCON printer thread, but netpoll_send_skb() explicitly disables hard interrupts around __netpoll_send_skb(): local_irq_save(flags); ret = __netpoll_send_skb(np, skb); local_irq_restore(flags); When direct transmission cannot complete, __netpoll_send_skb() calls skb_queue_tail() before interrupts are restored. Its spinlock can sleep on PREEMPT_RT despite the caller being a thread. The deferred skb is later handled by queue_process(), a workqueue callback. Its requeue path has the same IRQ-disabled locking: local_irq_save(flags); [...] HARD_TX_LOCK(dev, txq, smp_processor_id()); if (netif_xmit_frozen_or_stopped(txq) || !dev_xmit_complete(netpoll_start_xmit(skb, dev, txq))) { skb_queue_head(&npinfo->txq, skb); HARD_TX_UNLOCK(dev, txq); local_irq_restore(flags); [...] } Thanks, Karl