The Gemalto Cinterion PLS83-W modem (cdc_ether) is emitting confusing link up and down events when the WWAN interface is activated on the modem-side.
Interrupt URBs will in consecutive polls grab: * Link Connected * Link Disconnected * Link Connected
Where the last Connected is then a stable link state.
When the system is under load this may cause the unlink_urbs() work in __handle_link_change() to not complete before the next usbnet_link_change() call turns the carrier on again, allowing rx_submit() to queue new SKBs.
In that event the URB queue is filled faster than it can drain, ending up in a RCU stall:
rcu: INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 0-.... } 33108 jiffies s: 201 root: 0x1/. rcu: blocking rcu_node structures (internal RCU debug): Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0
Call trace: arch_local_irq_enable+0x4/0x8 local_bh_enable+0x18/0x20 __netdev_alloc_skb+0x18c/0x1cc rx_submit+0x68/0x1f8 [usbnet] rx_alloc_submit+0x4c/0x74 [usbnet] usbnet_bh+0x1d8/0x218 [usbnet] usbnet_bh_tasklet+0x10/0x18 [usbnet] tasklet_action_common+0xa8/0x110 tasklet_action+0x2c/0x34 handle_softirqs+0x2cc/0x3a0 __do_softirq+0x10/0x18 ____do_softirq+0xc/0x14 call_on_irq_stack+0x24/0x34 do_softirq_own_stack+0x18/0x20 __irq_exit_rcu+0xa8/0xb8 irq_exit_rcu+0xc/0x30 el1_interrupt+0x34/0x48 el1h_64_irq_handler+0x14/0x1c el1h_64_irq+0x68/0x6c _raw_spin_unlock_irqrestore+0x38/0x48 xhci_urb_dequeue+0x1ac/0x45c [xhci_hcd] unlink1+0xd4/0xdc [usbcore] usb_hcd_unlink_urb+0x70/0xb0 [usbcore] usb_unlink_urb+0x24/0x44 [usbcore] unlink_urbs.constprop.0.isra.0+0x64/0xa8 [usbnet] __handle_link_change+0x34/0x70 [usbnet] usbnet_deferred_kevent+0x1c0/0x320 [usbnet] process_scheduled_works+0x2d0/0x48c worker_thread+0x150/0x1dc kthread+0xd8/0xe8 ret_from_fork+0x10/0x20
Get around the problem by delaying the carrier on to the scheduled work.
This needs a new flag to keep track of the necessary action.
The carrier ok check cannot be removed as it remains required for the LINK_RESET event flow.
Fixes: 4b49f58fff00 ("usbnet: handle link change") Cc: stable@vger.kernel.org Signed-off-by: John Ernberg john.ernberg@actia.se
---
I've been testing this quite aggressively over a night, and seems equally stable to my first approach. I'm a little bit concerned that the bit stuff can now race (although much smaller) in the opposite direction, that a carrier off can occur between test_and_clear_bit() and the carrier on action in the handler. Leaving the carrier on when it shouldn't be.
v2: - target tree in patch description. - Drop Ming Lei from address list as their address bounces. - Rework solution based on feedback by Jakub (let me know if you want a Suggested-by tag, if we're keeping this direction)
v1: https://lore.kernel.org/netdev/20250710085028.1070922-1-john.ernberg@actia.s...
Tested on 6.12.20 and forward ported. Stack trace from 6.12.20. --- drivers/net/usb/usbnet.c | 11 ++++++++--- include/linux/usb/usbnet.h | 1 + 2 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c index c04e715a4c2a..bc1d8631ffe0 100644 --- a/drivers/net/usb/usbnet.c +++ b/drivers/net/usb/usbnet.c @@ -1122,6 +1122,9 @@ static void __handle_link_change(struct usbnet *dev) * tx queue is stopped by netcore after link becomes off */ } else { + if (test_and_clear_bit(EVENT_LINK_CARRIER_ON, &dev->flags)) + netif_carrier_on(dev->net); + /* submitting URBs for reading packets */ tasklet_schedule(&dev->bh); } @@ -2009,10 +2012,12 @@ EXPORT_SYMBOL(usbnet_manage_power); void usbnet_link_change(struct usbnet *dev, bool link, bool need_reset) { /* update link after link is reseted */ - if (link && !need_reset) - netif_carrier_on(dev->net); - else + if (link && !need_reset) { + set_bit(EVENT_LINK_CARRIER_ON, &dev->flags); + } else { + clear_bit(EVENT_LINK_CARRIER_ON, &dev->flags); netif_carrier_off(dev->net); + }
if (need_reset && link) usbnet_defer_kevent(dev, EVENT_LINK_RESET); diff --git a/include/linux/usb/usbnet.h b/include/linux/usb/usbnet.h index 0b9f1e598e3a..4bc6bb01a0eb 100644 --- a/include/linux/usb/usbnet.h +++ b/include/linux/usb/usbnet.h @@ -76,6 +76,7 @@ struct usbnet { # define EVENT_LINK_CHANGE 11 # define EVENT_SET_RX_MODE 12 # define EVENT_NO_IP_ALIGN 13 +# define EVENT_LINK_CARRIER_ON 14 /* This one is special, as it indicates that the device is going away * there are cyclic dependencies between tasklet, timer and bh * that must be broken
Hello:
This patch was applied to netdev/net.git (main) by Jakub Kicinski kuba@kernel.org:
On Wed, 23 Jul 2025 10:25:35 +0000 you wrote:
The Gemalto Cinterion PLS83-W modem (cdc_ether) is emitting confusing link up and down events when the WWAN interface is activated on the modem-side.
Interrupt URBs will in consecutive polls grab:
- Link Connected
- Link Disconnected
- Link Connected
[...]
Here is the summary with links: - [net,v2] net: usbnet: Avoid potential RCU stall on LINK_CHANGE event https://git.kernel.org/netdev/net/c/0d9cfc9b8cb1
You are awesome, thank you!
On 7/26/25 1:10 AM, patchwork-bot+netdevbpf@kernel.org wrote:
Here is the summary with links:
- [net,v2] net: usbnet: Avoid potential RCU stall on LINK_CHANGE event https://git.kernel.org/netdev/net/c/0d9cfc9b8cb1
I just got bitten by this commit after syncing with Linus' tree.
It breaks my laptop. RJ45 LAN cable cannot connect. After git bisect, it ends up with that commit.
ammarfaizi2@integral2:~/p/linux-block$ git bisect log git bisect start # bad: [ff82265b006e468df734a2d71f9110b73bd740f2] Merge branch 'master' into af/home (sync with mainline) git bisect bad ff82265b006e468df734a2d71f9110b73bd740f2 # good: [b0896d43221f7858491d59383f56dfe38e7fff34] Merge tag 'kvm-x86-vmx-6.17' of https://github.com/kvm-x86/linux into af/home git bisect good b0896d43221f7858491d59383f56dfe38e7fff34 # good: [5f5c9952b33cb4e8d25c70ef29f7a45cd26b6a9b] Merge tag 'powerpc-6.17-1' of git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux git bisect good 5f5c9952b33cb4e8d25c70ef29f7a45cd26b6a9b # bad: [8be4d31cb8aaeea27bde4b7ddb26e28a89062ebf] Merge tag 'net-next-6.17' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next git bisect bad 8be4d31cb8aaeea27bde4b7ddb26e28a89062ebf # good: [c2b93d6beca8526fb38ccc834def1c987afe24fc] eth: fbnic: Create ring buffer for firmware logs git bisect good c2b93d6beca8526fb38ccc834def1c987afe24fc # good: [077f7153fd2582874b0dec8c8fcd687677d0f4cc] gve: merge xdp and xsk registration git bisect good 077f7153fd2582874b0dec8c8fcd687677d0f4cc # good: [126d85fb040559ba6654f51c0b587d280b041abb] Merge tag 'wireless-next-2025-07-24' of https://git.kernel.org/pub/scm/linux/kernel/git/wireless/wireless-next git bisect good 126d85fb040559ba6654f51c0b587d280b041abb # good: [ecc383e5fe060f1aaad0e4e4ae36ad1c899e948d] Merge tag 'linux-can-next-for-6.17-20250725' of git://git.kernel.org/pub/scm/linux/kernel/git/mkl/linux-can-next git bisect good ecc383e5fe060f1aaad0e4e4ae36ad1c899e948d # bad: [c58c18be8850d58fd61b0480d2355df89ce7ee59] Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net git bisect bad c58c18be8850d58fd61b0480d2355df89ce7ee59 # good: [620e2392db235ba3b9e9619912aadb8cadee15e7] net: dsa: microchip: Disable PTP function of KSZ8463 git bisect good 620e2392db235ba3b9e9619912aadb8cadee15e7 # bad: [2764ab51d5f0e8c7d3b7043af426b1883e3bde1d] stmmac: xsk: fix negative overflow of budget in zerocopy mode git bisect bad 2764ab51d5f0e8c7d3b7043af426b1883e3bde1d # good: [4fc7885c3a98ec4450103aef874fb1d35920c7af] Merge branch 'mlx5e-misc-fixes-2025-07-23' git bisect good 4fc7885c3a98ec4450103aef874fb1d35920c7af # bad: [1bbb76a899486827394530916f01214d049931b3] neighbour: Fix null-ptr-deref in neigh_flush_dev(). git bisect bad 1bbb76a899486827394530916f01214d049931b3 # bad: [165a7f5db919ab68a45ae755cceb751e067273ef] net: dsa: microchip: Fix wrong rx drop MIB counter for KSZ8863 git bisect bad 165a7f5db919ab68a45ae755cceb751e067273ef # bad: [0d9cfc9b8cb17dbc29a98792d36ec39a1cf1395f] net: usbnet: Avoid potential RCU stall on LINK_CHANGE event git bisect bad 0d9cfc9b8cb17dbc29a98792d36ec39a1cf1395f # first bad commit: [0d9cfc9b8cb17dbc29a98792d36ec39a1cf1395f] net: usbnet: Avoid potential RCU stall on LINK_CHANGE event
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