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Fri, 15 Aug 2025 13:01:10 -0700 (PDT) Message-ID: <3dd73125-7f9b-405c-b5cd-0ab172014d00@gmail.com> Date: Fri, 15 Aug 2025 21:02:27 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: netconsole: HARDIRQ-safe -> HARDIRQ-unsafe lock order warning To: Breno Leitao , Jakub Kicinski Cc: Johannes Berg , Mike Galbraith , paulmck@kernel.org, LKML , netdev@vger.kernel.org, boqun.feng@gmail.com References: <20250814172326.18cf2d72@kernel.org> <3d20ce1b-7a9b-4545-a4a9-23822b675e0c@gmail.com> <20250815094217.1cce7116@kernel.org> Content-Language: en-US From: Pavel Begunkov In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 8/15/25 18:29, Breno Leitao wrote: > On Fri, Aug 15, 2025 at 09:42:17AM -0700, Jakub Kicinski wrote: >> On Fri, 15 Aug 2025 11:44:45 +0100 Pavel Begunkov wrote: >>> On 8/15/25 01:23, Jakub Kicinski wrote: >> >> I suspect disabling netconsole over WiFi may be the most sensible way out. > > I believe we might be facing a similar issue with virtio-net. > Specifically, any network adapter where TX is not safe to use in IRQ > context encounters this problem. > > If we want to keep netconsole enabled on all TX paths, a possible > solution is to defer the transmission work when netconsole is called > inside an IRQ. > > The idea is that netconsole first checks if it is running in an IRQ > context using in_irq(). If so, it queues the skb without transmitting it > immediately and schedules deferred work to handle the transmission > later. > > A rough implementation could be: > > static void send_udp(struct netconsole_target *nt, const char *msg, int len) { > > /* get the SKB that is already populated, with all the headers > * and ready to be sent > */ > struct sk_buff = netpoll_get_skb(&nt->np, msg, len); > > if (in_irq()) { It's not just irq handlers but any context that has irqs disabled, and since it's nested under irq-safe console_owner it'd need to always be deferred or somehow moved out of the console_owner critical section. Maybe there is printk lock trickery I don't understand, however. > skb_queue_tail(&np->delayed_queue, skb); > schedule_delayed_work(flush_delayed_queue, 0); > return; > } > > return __netpoll_send_skb(struct netpoll *np, struct sk_buff *skb) > } > > This approach does not require additional memory or extra data copying, > since copying from the printk buffer to the skb must be performed > regardless. > > The main drawback is a slight delay for messages sent from within an IRQ > context, though I believe such cases are infrequent. > > We could potentially also perform the flush from softirq context, which > would help reduce this latency further. -- Pavel Begunkov