Hello,

First, thank you for maintaining so many kernel versions so well!

I recently tested the v4.9 version and I had this warning:

[ 8.004728] ------------[ cut here ]------------ [ 8.005169] WARNING: CPU: 0 PID: 0 at /home/jenkins/slave/workspace/kernel_v4.9.x/kernelspace/kernel/softirq.c:165 __local_bh_enable_ip+0x66/0x80 [ 8.006397] Modules linked in: [ 8.006738] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.9.123 #2 [ 8.007297] Hardware name: Red Hat KVM, BIOS 1.11.0-2.el7 04/01/2014 [ 8.007841] ffff88001fc03a90 ffffffff811c9935 0000000000000000 0000000000000000 [ 8.008519] ffff88001fc03ad0 ffffffff81046d27 000000a50ca87c00 0000000000000200 [ 8.009186] ffff88000ca87c00 ffff88000f200000 ffff88000c5007c0 ffff88000b47f24e [ 8.009813] Call Trace: [ 8.010033] <IRQ> [ 8.010195] [<ffffffff811c9935>] dump_stack+0x63/0x8e [ 8.010636] [<ffffffff81046d27>] __warn+0xc7/0xf0 [ 8.011039] [<ffffffff81046e08>] warn_slowpath_null+0x18/0x20 [ 8.011503] [<ffffffff8104a686>] __local_bh_enable_ip+0x66/0x80 [ 8.012044] [<ffffffff814f8d55>] _raw_spin_unlock_bh+0x15/0x20 [ 8.012515] [<ffffffff8146ee87>] jtcp_rcv_established+0x227/0x2b0 [ 8.013050] [<ffffffff81444963>] tcp_v4_do_rcv+0x163/0x1f0 [ 8.013493] [<ffffffff81447b25>] tcp_v4_rcv+0xe85/0x10a0 [ 8.013957] [<ffffffff8146b23c>] ? nf_nat_ipv4_fn+0x19c/0x1e0 [ 8.014431] [<ffffffff8146e8b0>] ? iptable_nat_ipv4_fn+0x20/0x20 [ 8.015013] [<ffffffff8141fa8f>] ip_local_deliver_finish+0x9f/0x140 [ 8.015517] [<ffffffff81420090>] ip_local_deliver+0xc0/0xd0 [ 8.016027] [<ffffffff8141f9f0>] ? inet_del_offload+0x40/0x40 [ 8.016490] [<ffffffff8141fccb>] ip_rcv_finish+0x19b/0x350 [ 8.017028] [<ffffffff81420353>] ip_rcv+0x2b3/0x460 [ 8.017425] [<ffffffff8141fb30>] ? ip_local_deliver_finish+0x140/0x140 [ 8.017990] [<ffffffff813b5bde>] __netif_receive_skb_core+0x47e/0x840 [ 8.018508] [<ffffffff8144cff1>] ? tcp4_gro_receive+0x131/0x1b0 [ 8.019029] [<ffffffff8145c2b1>] ? inet_gro_receive+0x231/0x2a0 [ 8.019507] [<ffffffff813b8258>] __netif_receive_skb+0x18/0x60 [ 8.020053] [<ffffffff813b82c8>] netif_receive_skb_internal+0x28/0x90 [ 8.020570] [<ffffffff813b8b48>] napi_gro_receive+0x78/0xa0 [ 8.021064] [<ffffffff81332c28>] virtnet_receive+0x1f8/0x890 [ 8.021520] [<ffffffff813333b8>] virtnet_poll+0x18/0x80 [ 8.021986] [<ffffffff813b9cc6>] net_rx_action+0xf6/0x2c0 [ 8.022422] [<ffffffff8104a44c>] __do_softirq+0xcc/0x1e0 [ 8.022874] [<ffffffff8104a767>] irq_exit+0x67/0x70 [ 8.023290] [<ffffffff81019be1>] do_IRQ+0x51/0xe0 [ 8.023695] [<ffffffff814f99d6>] common_interrupt+0x96/0x96 [ 8.024189] <EOI> [ 8.024351] [<ffffffff814f8b30>] ? __sched_text_end+0x3/0x3 [ 8.024830] [<ffffffff814f8bc6>] ? native_safe_halt+0x6/0x10 [ 8.025307] [<ffffffff814f8b39>] default_idle+0x9/0x10 [ 8.025742] [<ffffffff81021210>] arch_cpu_idle+0x10/0x20 [ 8.026190] [<ffffffff814f8c4e>] default_idle_call+0x1e/0x30 [ 8.026813] [<ffffffff81077621>] cpu_startup_entry+0xe1/0x1d0 [ 8.027397] [<ffffffff814f453d>] rest_init+0x6d/0x70 [ 8.027832] [<ffffffff8188f0dc>] start_kernel+0x4d0/0x4dd [ 8.028360] [<ffffffff8188e9fa>] ? set_init_arg+0x55/0x55 [ 8.028821] [<ffffffff8188e120>] ? early_idt_handler_array+0x120/0x120 [ 8.029372] [<ffffffff8188e599>] x86_64_start_reservations+0x2a/0x2c [ 8.029910] [<ffffffff8188e681>] x86_64_start_kernel+0xe6/0xf3 [ 8.030412] ---[ end trace 5826c2ad94ee574a ]---

After a quick search, Christoph found that this kind of call trace had already been reported by Intel's kernel test bot: https://lkml.org/lkml/2017/2/19/251

According to this bot, it seems that the following commit caused the warning:

e70ac171658679ecf6bea4bbd9e9325cd6079d2b (tcp: tcp_probe: use spin_lock_bh())

Note that this commit has been backported from v4.10 to v4.9.33.

The next day, Eric Dumazet, the author of the first patch, proposed a second one which "simply" reverts this commit:

29869d66870a715177bfb505f66a7e0e8bcc89c3 (tcp: Revert "tcp: tcp_probe: use spin_lock_bh()")

You can find the discussions about this patch here, including a question from Eric not being sure why it is not needed: https://patchwork.ozlabs.org/patch/730560/

After having applied this patch (the reverted commit - 29869d66870a) on the v4.9 tree, I confirm I no longer have the warning mentioned at the beginning of this email.

I don't think many people are still using tcp probe -- recently removed in v4.16 -- but it could be nice if someone could backport the commit 29869d66870a in v4.9.y tree to avoid extra warnings.

Again, thank you for your very nice work!

Best regards, Matt