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[2a02:3100:acba:a601:4494:3582:465b:eab]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4887a30b1dfsm26283196f8f.4.2026.09.27.23.42.49 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Sun, 27 Sep 2026 23:42:52 -0700 (PDT) From: Karl Mehltretter To: netdev@vger.kernel.org Cc: Karl Mehltretter , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Stephen Hemminger , linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, stable@vger.kernel.org Subject: [PATCH net 0/2] netpoll: fix PREEMPT_RT deferred transmit locking Date: Mon, 28 Sep 2026 08:42:37 +0200 Message-Id: <20260928064239.32456-1-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit netpoll's deferred transmit path can acquire two sleeping locks with hard interrupts disabled on PREEMPT_RT. One is the sk_buff_head lock used by the sender and worker. The other is the device transmit lock used by the worker. They produce independent atomic-sleep reports. Patch 1 gives the deferred skb queue a dedicated raw lock. Patch 2 makes the worker try the device transmit lock and defer the skb again when the lock is busy. It uses the queue helper added by patch 1. Testing: - Arm64 and x86_64 PREEMPT_RT W=1 builds passed. - An unpatched Pi 400 emitted the queue-lock report five times during a one-hour run. Ten boots with the complete fix passed, including one 3,606-second run. Every trigger and post-trigger marker arrived, Wi-Fi remained usable, and neither atomic-sleep warning occurred. - An arm64 PREEMPT_RT QEMU control reproduced both warnings under forced transmit backpressure. Three boots with the complete fix passed. - An x86_64 QEMU control reproduced the queue-lock warning on PREEMPT_RT. Three fixed RT boots passed. The non-RT control and treatment each passed three boots. Karl Mehltretter (2): netpoll: use a raw lock for the deferred transmit queue netpoll: avoid blocking on the transmit lock in queue_process include/linux/netpoll.h | 1 + net/core/netpoll.c | 83 +++++++++++++++++++++++++++++++++++++---- 2 files changed, 77 insertions(+), 7 deletions(-) base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7 -- 2.53.0