Subject: Re: [syzbot] [net?] possible deadlock in rlb_choose_channel (2)

From: Jay Vosburgh <[email-masked]>
Date: Wed, 13 May 2026 16:41:28 +0200

syzbot <[email-masked]> wrote:

>Hello,
>
>syzbot found the following issue on:
>
>HEAD commit:    c21b90f77687 x86/CPU/AMD: Prevent improper isolation of sh..
>git tree:       upstream
>console output: https://syzkaller.appspot.com/x/log.txt?x=10ec7dba580000
>kernel config:  https://syzkaller.appspot.com/x/.config?x=4caf64b1ee83dac0
>dashboard link: https://syzkaller.appspot.com/bug?extid=1db58dbbccbf93c65c83
>compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>
>Unfortunately, I don't have any reproducer for this issue yet.
>
>Downloadable assets:
>disk image: https://storage.googleapis.com/syzbot-assets/2f3edabe3b67/disk-c21b90f7.raw.xz
>vmlinux: https://storage.googleapis.com/syzbot-assets/539b63753e79/vmlinux-c21b90f7.xz
>kernel image: https://storage.googleapis.com/syzbot-assets/48e6e7cbc4ca/bzImage-c21b90f7.xz
>
>IMPORTANT: if you fix the issue, please add the following tag to the commit:
>Reported-by: [email-masked]
>
>ip6_tunnel: ip6tnl1 xmit: Local address not yet configured!
>ip6_tunnel: ip6tnl1 xmit: Local address not yet configured!
>============================================
>WARNING: possible recursive locking detected
>syzkaller #0 Tainted: G             L     
>--------------------------------------------
>kworker/u8:3/47 is trying to acquire lock:
>ffff88807a618e98 (&bond->mode_lock){+.-.}-{3:3}, at: spin_lock include/linux/spinlock.h:342 [inline]
>ffff88807a618e98 (&bond->mode_lock){+.-.}-{3:3}, at: rlb_choose_channel+0x37/0x19a0 drivers/net/bonding/bond_alb.c:562
>
>but task is already holding lock:
>ffff88807ffa0e98 (&bond->mode_lock){+.-.}-{3:3}, at: spin_lock_bh include/linux/spinlock.h:348 [inline]
>ffff88807ffa0e98 (&bond->mode_lock){+.-.}-{3:3}, at: rlb_update_rx_clients drivers/net/bonding/bond_alb.c:466 [inline]
>ffff88807ffa0e98 (&bond->mode_lock){+.-.}-{3:3}, at: bond_alb_monitor+0xe8a/0x17e0 drivers/net/bonding/bond_alb.c:1618
>
>other info that might help us debug this:
> Possible unsafe locking scenario:
>
>       CPU0
>       ----
>  lock(&bond->mode_lock);
>  lock(&bond->mode_lock);
>
> *** DEADLOCK ***
>
> May be due to missing lock nesting notation
>
>7 locks held by kworker/u8:3/47:
> #0: ffff8880516b7140 ((wq_completion)bond5#2){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3277 [inline]
> #0: ffff8880516b7140 ((wq_completion)bond5#2){+.+.}-{0:0}, at: process_scheduled_works+0xa35/0x1860 kernel/workqueue.c:3385
> #1: ffffc90000b77c40 ((work_completion)(&(&bond->alb_work)->work)){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3278 [inline]
> #1: ffffc90000b77c40 ((work_completion)(&(&bond->alb_work)->work)){+.+.}-{0:0}, at: process_scheduled_works+0xa70/0x1860 kernel/workqueue.c:3385
> #2: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
> #2: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
> #2: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: bond_alb_monitor+0xf8/0x17e0 drivers/net/bonding/bond_alb.c:1546
> #3: ffff88807ffa0e98 (&bond->mode_lock){+.-.}-{3:3}, at: spin_lock_bh include/linux/spinlock.h:348 [inline]
> #3: ffff88807ffa0e98 (&bond->mode_lock){+.-.}-{3:3}, at: rlb_update_rx_clients drivers/net/bonding/bond_alb.c:466 [inline]
> #3: ffff88807ffa0e98 (&bond->mode_lock){+.-.}-{3:3}, at: bond_alb_monitor+0xe8a/0x17e0 drivers/net/bonding/bond_alb.c:1618
> #4: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
> #4: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
> #4: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: arp_xmit+0x23/0x270 net/ipv4/arp.c:663
> #5: ffffffff8e95cdc0 (rcu_read_lock_bh){....}-{1:3}, at: local_bh_disable include/linux/bottom_half.h:20 [inline]
> #5: ffffffff8e95cdc0 (rcu_read_lock_bh){....}-{1:3}, at: rcu_read_lock_bh include/linux/rcupdate.h:891 [inline]
> #5: ffffffff8e95cdc0 (rcu_read_lock_bh){....}-{1:3}, at: __dev_queue_xmit+0x2b6/0x3950 net/core/dev.c:4791
> #6: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
> #6: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
> #6: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: bond_start_xmit+0xb4/0x1900 drivers/net/bonding/bond_main.c:5591
>
>stack backtrace:
>CPU: 0 UID: 0 PID: 47 Comm: kworker/u8:3 Tainted: G             L      syzkaller #0 PREEMPT(full) 
>Tainted: [L]=SOFTLOCKUP
>Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
>Workqueue: bond5 bond_alb_monitor
>Call Trace:
> <TASK>
> dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
> print_deadlock_bug+0x279/0x290 kernel/locking/lockdep.c:3041
> check_deadlock kernel/locking/lockdep.c:3093 [inline]
> validate_chain kernel/locking/lockdep.c:3895 [inline]
> __lock_acquire+0x253f/0x2cf0 kernel/locking/lockdep.c:5237
> lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
> __raw_spin_lock include/linux/spinlock_api_smp.h:158 [inline]
> _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:158
> spin_lock include/linux/spinlock.h:342 [inline]
> rlb_choose_channel+0x37/0x19a0 drivers/net/bonding/bond_alb.c:562
> rlb_arp_xmit drivers/net/bonding/bond_alb.c:680 [inline]
> bond_xmit_alb_slave_get+0x1071/0x20a0 drivers/net/bonding/bond_alb.c:1493
> bond_alb_xmit+0x24/0x40 drivers/net/bonding/bond_alb.c:1528
> __bond_start_xmit drivers/net/bonding/bond_main.c:5569 [inline]
> bond_start_xmit+0x6a2/0x1900 drivers/net/bonding/bond_main.c:5593
> __netdev_start_xmit include/linux/netdevice.h:5368 [inline]
> netdev_start_xmit include/linux/netdevice.h:5377 [inline]
> xmit_one net/core/dev.c:3888 [inline]
> dev_hard_start_xmit+0x2cd/0x830 net/core/dev.c:3904
> __dev_queue_xmit+0x14d9/0x3950 net/core/dev.c:4870
> NF_HOOK+0x33a/0x3c0 include/linux/netfilter.h:-1
> arp_xmit+0x16c/0x270 net/ipv4/arp.c:665
> rlb_update_client+0x2a8/0x6b0 drivers/net/bonding/bond_alb.c:455
> rlb_update_rx_clients drivers/net/bonding/bond_alb.c:473 [inline]
> bond_alb_monitor+0xf6a/0x17e0 drivers/net/bonding/bond_alb.c:1618

	Just looking at the stack, I suspect that this is either a false
positive, or the NF_HOOK action (a netfilter rule) is reinjecting the
ARP packet in to the same bond that created it.

	If the packet is being reinjected to the same interface that
generated it in rlb_update_client, then I believe the above would be the
expected behavior.

	On the other hand, if the network configuration is nested bonds,
then the rlb_arp_xmit -> rlb_choose_channel call path above would be
operating on a different instance of the bond->mode_lock, and would not
actually deadlock.

	-J

> process_one_work kernel/workqueue.c:3302 [inline]
> process_scheduled_works+0xb5d/0x1860 kernel/workqueue.c:3385
> worker_thread+0xa53/0xfc0 kernel/workqueue.c:3466
> kthread+0x388/0x470 kernel/kthread.c:436
> ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> </TASK>
>
>
>---
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---
	-Jay Vosburgh, [email-masked]