Hi,

I was playing around with the tracers I encountered a lockdep
warning when the system is using threaded IRQs. I can trigger it by
adding threadirqs to the kernel options and then do a 

    echo preemptoff > /sys/kernel/debug/tracing/current_tracer


[  206.610079] ------------[ cut here ]------------
[  206.610122] WARNING: CPU: 0 PID: 3 at kernel/locking/lockdep.c:3574 check_flags.part.36+0x1bc/0x210()
[  206.610166] DEBUG_LOCKS_WARN_ON(current->softirqs_enabled)
[  206.610192] Modules linked in:
[  206.610224]  x86_pkg_temp_thermal coretemp kvm_intel kvm crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel iTCO_wdt iTCO_vendor_support ipmi_devintf dcdbas joydev tpm_tis pcspkr acpi_pad ipmi_si ipmi_msghandler acpi_power_meter wmi mei_me tpm sb_edac shpchp edac_core mei lpc_ich mfd_core nfsd auth_rpcgss nfs_acl lockd grace sunrpc xfs libcrc32c mgag200 drm_kms_helper ttm drm ixgbe igb mdio vxlan ip6_udp_tunnel udp_tunnel megaraid_sas ptp pps_core dca i2c_algo_bit
[  206.610892] CPU: 0 PID: 3 Comm: ksoftirqd/0 Not tainted 4.1.0-rc4 #99
[  206.610952] Hardware name: Dell Inc. PowerEdge R820/066N7P, BIOS 2.0.20 01/16/2014
[  206.611021]  ffffffff8186e017 000000005a61066b ffff881fb11d3b28 ffffffff8186e017
[  206.611118]  0000000000000000 ffff881fb11d3b80 ffff881fb11d3b68 ffffffff810af8fa
[  206.611211]  ffff881fb11d3b88 ffffffff81e6cb58 0000000000000000 0000000000000000
[  206.611302] Call Trace:
[  206.611338]  [<ffffffff8186e017>] ? dump_stack+0x4f/0x7b
[  206.611392]  [<ffffffff8186e017>] dump_stack+0x4f/0x7b
[  206.611448]  [<ffffffff810af8fa>] warn_slowpath_common+0x8a/0xc0
[  206.611507]  [<ffffffff810af985>] warn_slowpath_fmt+0x55/0x70
[  206.611565]  [<ffffffff810af935>] ? warn_slowpath_fmt+0x5/0x70
[  206.611623]  [<ffffffff811126fc>] check_flags.part.36+0x1bc/0x210
[  206.611682]  [<ffffffff81113479>] lock_acquire+0x199/0x2d0
[  206.611738]  [<ffffffff8187920b>] ? _raw_spin_lock_irqsave+0x2b/0xa0
[  206.611800]  [<ffffffff810e5925>] ? preempt_count_add+0x5/0xc0
[  206.611860]  [<ffffffff81879239>] _raw_spin_lock_irqsave+0x59/0xa0
[  206.611922]  [<ffffffff811b773e>] ? check_critical_timing+0xae/0x170
[  206.611982]  [<ffffffff818791e5>] ? _raw_spin_lock_irqsave+0x5/0xa0
[  206.612043]  [<ffffffff811b773e>] check_critical_timing+0xae/0x170
[  206.612102]  [<ffffffff810b606c>] ? __do_softirq+0x24c/0x6b0
[  206.612158]  [<ffffffff810b434b>] ? __local_bh_enable+0x3b/0x70
[  206.612213]  [<ffffffff810b606c>] ? __do_softirq+0x24c/0x6b0
[  206.612266]  [<ffffffff810b434b>] ? __local_bh_enable+0x3b/0x70
[  206.612322]  [<ffffffff811b8401>] trace_preempt_on+0x101/0x120
[  206.612376]  [<ffffffff810e59e5>] ? preempt_count_sub+0x5/0xf0
[  206.612431]  [<ffffffff810e5a8b>] preempt_count_sub+0xab/0xf0
[  206.612485]  [<ffffffff810b434b>] ? __local_bh_enable+0x3b/0x70
[  206.612540]  [<ffffffff810b434b>] __local_bh_enable+0x3b/0x70
[  206.612595]  [<ffffffff810b606c>] ? __do_softirq+0x24c/0x6b0
[  206.612649]  [<ffffffff810b606c>] __do_softirq+0x24c/0x6b0
[  206.612703]  [<ffffffff810b64f5>] run_ksoftirqd+0x25/0x70
[  206.614370]  [<ffffffff810db453>] smpboot_thread_fn+0x193/0x2a0
[  206.616013]  [<ffffffff810db2c0>] ? sort_range+0x30/0x30
[  206.616948]  [<ffffffff810d71f4>] kthread+0x104/0x120
[  206.617733]  [<ffffffff81873293>] ? wait_for_completion+0xd3/0x140
[  206.618512]  [<ffffffff810d70f0>] ? kthread_create_on_node+0x260/0x260
[  206.619281]  [<ffffffff818798a2>] ret_from_fork+0x42/0x70
[  206.620054]  [<ffffffff810d70f0>] ? kthread_create_on_node+0x260/0x260
[  206.620831] ---[ end trace 9e22068fb8699756 ]---
[  206.621607] irq event stamp: 636192
[  206.622362] hardirqs last  enabled at (636190): [<ffffffff818787e5>] _raw_spin_unlock_irqrestore+0x65/0x90
[  206.623140] hardirqs last disabled at (636191): [<ffffffff810b5ffe>] __do_softirq+0x1de/0x6b0
[  206.623904] softirqs last  enabled at (636192): [<ffffffff810b606c>] __do_softirq+0x24c/0x6b0
[  206.624653] softirqs last disabled at (635925): [<ffffffff810b64f5>] run_ksoftirqd+0x25/0x70

There is no warning without threadirqs. 

The same warning can also be triggered inside KVM. I tried to
bisect it but kernel version before v3.15 didn't boot due to
some systemd dependencies.

Any ideas?

cheers,
daniel