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Tue, 02 Jun 2026 11:53:37 -0700 (PDT) Date: Tue, 2 Jun 2026 11:53:30 -0700 From: Stanislav Fomichev To: Breno Leitao Cc: michael.chan@broadcom.com, pavan.chebbi@broadcom.com, stfomichev@gmail.com, kuba@kernel.org, kernel-team@meta.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: bnxt_en: suspicious RCU usage in bnxt_fw_reset_task() qdisc path in 7.1-rc6 Message-ID: References: Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On 06/02, Stanislav Fomichev wrote: > On 06/02, Breno Leitao wrote: > > I am hitting a "suspicious RCU usage" lockdep splat from bnxt_en > > during a firmware reset on v7.1-rc6 (e43ffb69e043) with PROVE_RCU > > and PROVE_LOCKING enabled. > > > > The firmware reset path re-opens the device from bnxt_fw_reset_task() > > holding only the netdev instance lock (&dev->lock). bnxt_open() -> > > __bnxt_open_nic() -> bnxt_set_real_num_queues() -> > > netif_set_real_num_tx_queues() then walks the qdisc tree in > > dev_qdisc_change_real_num_tx(), which still dereferences dev->qdisc > > with rtnl_dereference() and therefore expects rtnl_lock to be held: > > > > void dev_qdisc_change_real_num_tx(struct net_device *dev, > > unsigned int new_real_tx) > > { > > struct Qdisc *qdisc = rtnl_dereference(dev->qdisc); > > ... > > } > > > > Since only the instance lock is held in this path, lockdep complains. > > > > Splat > > ----- > > > > ============================= > > WARNING: suspicious RCU usage > > 7.1.0 #1 Tainted: G E > > ----------------------------- > > net/sched/sch_generic.c:1416 suspicious rcu_dereference_protected() usage! > > > > other info that might help us debug this: > > > > rcu_scheduler_active = 2, debug_locks = 1 > > 3 locks held by kworker/u208:1/13: > > #0: ((wq_completion)bnxt_pf_wq){+.+.}-{0:0}, at: process_scheduled_works+0x8f7/0x13b0 > > #1: ((work_completion)(&(&bp->fw_reset_task)->work)){+.+.}-{0:0}, at: process_scheduled_works+0x917/0x13b0 > > #2: (&dev->lock){+.+.}-{4:4}, at: bnxt_fw_reset_task+0x7ed/0x1e80 > > stack backtrace: > > CPU: 38 UID: 0 PID: 13 Comm: kworker/u208:1 Tainted: G E > > Hardware name: Wiwynn Delta Lake MP/Delta Lake-Class1, BIOS Y3DL405 11/21/2025 > > Workqueue: bnxt_pf_wq bnxt_fw_reset_task > > > > dump_stack_lvl+0x69/0xa0 > > lockdep_rcu_suspicious+0x13f/0x1d0 > > dev_qdisc_change_real_num_tx+0x54/0xe0 > > netif_set_real_num_tx_queues+0x4ed/0xa80 > > __bnxt_open_nic+0x9cb/0x3490 > > ? bnxt_hwrm_if_change+0x4fd/0x620 > > bnxt_open+0x1cb/0x370 > > bnxt_fw_reset_task+0x80d/0x1e80 > > process_scheduled_works+0x9c1/0x13b0 > > worker_thread+0x90d/0xd20 > > kthread+0x320/0x3f0 > > ret_from_fork+0x2b6/0xb00 > > ret_from_fork_asm+0x11/0x20 > > > > > > Bisect / analysis > > ----------------- > > > > This looks like a regression from: > > > > 850d9248d2ea ("Revert "bnxt_en: bring back rtnl_lock() in the > > bnxt_open() path"") > > > > That revert dropped rtnl_lock() from the bnxt_open() paths and left > > bnxt_fw_reset_task() holding only the instance lock around bnxt_open(). > > > > I can send a patch restoring rtnl_lock() around the bnxt_open() call in > > bnxt_fw_reset_task() (re-taking rtnl before the instance lock, matching the > > pre-revert code), but I wanted to report it first to make sure I am in the > > right direction. > > > > Please let me know how you would like to proceed. > > I hope (but need to verify) that at this point most of the paths that assign > qdisc are under ops lock as well. So the alternative might be to use > netdev_ops_lock_dereference from Jakub's recent > https://lore.kernel.org/netdev/20260528231637.251822-1-kuba@kernel.org/t/#m3de56e1d53f86b2c1b12ca29d1617dd6635e78cb Agree with Jakub, this is more than just the assignment :-(