"BUG: scheduling while atomic" on 5.4 kernels with PREEMPT_RT

[Erik Schuitema <erik@essd.nl>]

Hi,

I ran into an issue with Wireguard on Linux 5.4 kernels with the 
PREEMPT_RT patch, on Ubuntu 18.04. I tried kernels 5.4.47-rt28 and 
5.4.82-rt45.
Everything is fine until I send actual data to the machine through scp, 
resulting in the kernel log below stating "BUG: scheduling while 
atomic".
I tried both the latest Ubuntu package (with wireguard-dkms version 
1.0.20201112) as well as compiling the kernel module from the latest 
source from the wireguard-linux-compat repo, with the same result.

Since the call trace mentions kernel_fpu_begin, I looked at the code and 
the issue seems to occur while using SIMD for packet decryption.

When I forcibly disable SIMD with this simple bypass:
  static inline void simd_get(simd_context_t *ctx)
  {
-    *ctx = !IS_ENABLED(CONFIG_PREEMPT_RT_BASE) && may_use_simd() ? 
HAVE_FULL_SIMD : HAVE_NO_SIMD;
+    *ctx = HAVE_NO_SIMD;
  }
indeed everything works fine again (ignoring the performance hit).

I was unable to further pinpoint the issue, unfortunately.
Any idea what might be the cause?

Best regards,
Erik Schuitema

=== kernel log ===

000: BUG: scheduling while atomic: kworker/0:1/15/0x00000002
000: Modules linked in: wireguard(E) ip6_udp_tunnel udp_tunnel 
intel_rapl_msr 8250_dw nls_iso8859_1 intel_rapl_common 
x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm irqbypass 
intel_cstate intel_rapl_perf joydev input_leds wmi_bmof 
intel_wmi_thunderbolt serio_raw snd_hda_codec_hdmi mei_me mei 
snd_hda_intel snd_intel_nhlt snd_hda_codec snd_hda_core snd_hwdep 
intel_lpss_pci snd_pcm intel_lpss snd_timer idma64 intel_pch_thermal 
virt_dma snd soundcore mac_hid acpi_pad ip6t_REJECT nf_reject_ipv6 
nf_log_ipv6 sch_fq_codel xt_hl ip6t_rt ipt_REJECT nf_reject_ipv4 
nf_log_ipv4 nf_log_common xt_LOG xt_limit xt_tcpudp xt_addrtype 
xt_conntrack ib_iser ip6table_filter rdma_cm ip6_tables iw_cm ib_cm 
nf_conntrack_netbios_ns nf_conntrack_broadcast nf_nat_ftp nf_nat ib_core 
nf_conntrack_ftp iscsi_tcp libiscsi_tcp nf_conntrack libiscsi 
nf_defrag_ipv6 scsi_transport_iscsi nf_defrag_ipv4 iptable_filter 
ip_tables x_tables autofs4 btrfs zstd_compress algif_skcipher af_alg 
dm_crypt raid10
000:  raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx 
xor raid6_pq libcrc32c raid1 raid0 multipath linear hid_logitech_hidpp 
hid_logitech_dj hid_generic usbhid hid amdgpu i915 gpu_sched ttm 
crct10dif_pclmul crc32_pclmul ghash_clmulni_intel drm_kms_helper 
syscopyarea nvme sysfillrect sysimgblt fb_sys_fops aesni_intel igb 
e1000e crypto_simd dca cryptd glue_helper ptp psmouse pps_core nvme_core 
i2c_algo_bit drm wmi video pinctrl_sunrisepoint
000: Preemption disabled at:
000: [<ffffffff87442203>] kernel_fpu_begin+0x13/0xd0
000: CPU: 0 PID: 15 Comm: kworker/0:1 Tainted: G            E     
5.4.47-rt28 #1
000: Hardware name: Intel(R) Client Systems NUC8i7HVK/NUC8i7HVB, BIOS 
HNKBLi70.86A.0064.2020.1028.1438 10/28/2020
000: Workqueue: wg-crypt-wg0 wg_packet_decrypt_worker [wireguard]
000: Call Trace:
000:  dump_stack+0x6f/0x95
000:  ? kernel_fpu_begin+0x13/0xd0
000:  __schedule_bug+0x78/0xc0
000:  __schedule+0x5f3/0x8b0
000:  ? task_blocks_on_rt_mutex+0x17c/0x350
000:  schedule+0x3d/0xe0
000:  rt_spin_lock_slowlock_locked+0x103/0x2e0
000:  rt_spin_lock_slowlock+0x57/0x90
000:  rt_spin_lock+0x44/0x50
000:  ? wg_packet_decrypt_worker+0xea/0x1c0 [wireguard]
000:  wg_packet_decrypt_worker+0xff/0x1c0 [wireguard]
000:  process_one_work+0x1ee/0x4d0
000:  worker_thread+0x34/0x3f0
000:  kthread+0x121/0x140
000:  ? process_one_work+0x4d0/0x4d0
000:  ? kthread_park+0x90/0x90
000:  ret_from_fork+0x35/0x40
000: ------------[ cut here ]------------