[BUG 6.18.2] Null Pointer Exception in Fair Scheduler

[BUG 6.18.2] Null Pointer Exception in Fair Scheduler

[Dylan E.]

Hello,

When booting into the v6.18.2 tagged kernel from linux-stable, I get the 
following
stack trace while booting into the system every 1 in 5 boots or so, 
usually during
fsck or early systemd service initialization:

---
BUG: kernel NULL pointer dereference, address: 0000000000000051
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP
CPU: 0 UID: 0 PID: 15 Comm: rcu_preempt Not tainted 6.18.2 #2 PREEMPT(full)
Hardware name:  /SKYBAY, BIOS 5.12 06/27/2017
RIP: 0010:pick_task_fair+0x57/0x160
Code: 66 90 66 90 48 8b 5d 50 48 85 db 74 10 48 8b 73 70 48 89 ef e8 3a 
74 ff ff 85 c0 75 71 be 01 00 00 00 48 89 ef e8 29 a5 ff ff <80> 78 51 
00 48 89 c3 0f 85 80 00 00 00 48 85 c0 0f 84 87 00 00 00
RSP: 0000:ffffc900000d3cf8 EFLAGS: 00010086
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000800
RDX: fffffc02295d3c00 RSI: 0000000000000800 RDI: 0000000002edc4f2
RBP: ffff888108f13000 R08: 0000000000000400 R09: 0000000000000002
R10: 0000000000000260 R11: ffff888108b74200 R12: ffff888265c2cd00
R13: 0000000000000000 R14: ffff888265c2cd80 R15: ffffffff827c6fa0
FS:  0000000000000000(0000) GS:ffff8882e2724000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000051 CR3: 00000001110a5003 CR4: 00000000003706f0
Call Trace:
  <TASK>
  pick_next_task_fair+0x1d/0x3d0
  __schedule+0x1ee/0x10c0
  ? lock_timer_base+0x6d/0x90
  ? rcu_gp_cleanup+0x560/0x560
  schedule+0x23/0xc0
  schedule_timeout+0x6e/0xe0
  ? hrtimers_cpu_dying+0x1b0/0x1b0
  rcu_gp_fqs_loop+0xfb/0x510
  rcu_gp_kthread+0xcd/0x160
  kthread+0xf5/0x1e0
  ? kthreads_online_cpu+0x100/0x100
  ? kthreads_online_cpu+0x100/0x100
  ret_from_fork+0x114/0x140
  ? kthreads_online_cpu+0x100/0x100
  ret_from_fork_asm+0x11/0x20
  </TASK>
Modules linked in: i915 drm_buddy intel_gtt drm_client_lib 
drm_display_helper drm_kms_helper igb cec dca rc_core i2c_algo_bit ttm 
agpgart e1000e serio_raw hwmon drm mei_wdt i2c_core intel_oc_wdt video wmi
CR2: 0000000000000051
---[ end trace 0000000000000000 ]---
RIP: 0010:pick_task_fair+0x57/0x160
Code: 66 90 66 90 48 8b 5d 50 48 85 db 74 10 48 8b 73 70 48 89 ef e8 3a 
74 ff ff 85 c0 75 71 be 01 00 00 00 48 89 ef e8 29 a5 ff ff <80> 78 51 
00 48 89 c3 0f 85 80 00 00 00 48 85 c0 0f 84 87 00 00 00
RSP: 0000:ffffc900000d3cf8 EFLAGS: 00010086
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000800
RDX: fffffc02295d3c00 RSI: 0000000000000800 RDI: 0000000002edc4f2
RBP: ffff888108f13000 R08: 0000000000000400 R09: 0000000000000002
R10: 0000000000000260 R11: ffff888108b74200 R12: ffff888265c2cd00
R13: 0000000000000000 R14: ffff888265c2cd80 R15: ffffffff827c6fa0
FS:  0000000000000000(0000) GS:ffff8882e2724000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000051 CR3: 00000001110a5003 CR4: 00000000003706f0
Kernel panic - not syncing: Fatal exception
Shutting down cpus with NMI
Kernel Offset: disabled
Rebooting in 30 seconds..
---

I can't seem to reproduce this issue with the v6.18.1 tagged 
linux-stable build, and
after bisecting between v6.18.1 and v6.18.2, I land on this commit 
(which is clearly
not the problem):

d911fa97dab3ba026a8b96bb7f833d007b7fc4e1 | wifi: ath12k: fix VHT MCS 
assignment

I don't have any ath12k radios in my system, but I do have 1 ath9k and 2 
ath10k
radios. A little up the tree I see this patch which *could* be related, 
but I lack
the knowledge to know:

b1497ea246396962156b63d5c568a16d6e32de0b | wifi: ath10k: move recovery 
check logic into a new work

Let me know if there's any more info that's needed or additional steps I 
can take to
further diagnose the bug.

Hardware Info:
- 1x Ath9k, 2x Ath10k, 1x MT7916
- Intel Core i5-6500TE

-- Dylan

Re: [BUG 6.18.2] Null Pointer Exception in Fair Scheduler

[Salvatore Bonaccorso]

Hi Dylan,

On Wed, Dec 31, 2025 at 12:00:07PM -0800, Dylan E. wrote:
> Hello,
> 
> When booting into the v6.18.2 tagged kernel from linux-stable, I get the
> following
> stack trace while booting into the system every 1 in 5 boots or so, usually
> during
> fsck or early systemd service initialization:
> 
> ---
> BUG: kernel NULL pointer dereference, address: 0000000000000051
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> Oops: Oops: 0000 [#1] SMP
> CPU: 0 UID: 0 PID: 15 Comm: rcu_preempt Not tainted 6.18.2 #2 PREEMPT(full)
> Hardware name:  /SKYBAY, BIOS 5.12 06/27/2017
> RIP: 0010:pick_task_fair+0x57/0x160
> Code: 66 90 66 90 48 8b 5d 50 48 85 db 74 10 48 8b 73 70 48 89 ef e8 3a 74
> ff ff 85 c0 75 71 be 01 00 00 00 48 89 ef e8 29 a5 ff ff <80> 78 51 00 48 89
> c3 0f 85 80 00 00 00 48 85 c0 0f 84 87 00 00 00
> RSP: 0000:ffffc900000d3cf8 EFLAGS: 00010086
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000800
> RDX: fffffc02295d3c00 RSI: 0000000000000800 RDI: 0000000002edc4f2
> RBP: ffff888108f13000 R08: 0000000000000400 R09: 0000000000000002
> R10: 0000000000000260 R11: ffff888108b74200 R12: ffff888265c2cd00
> R13: 0000000000000000 R14: ffff888265c2cd80 R15: ffffffff827c6fa0
> FS:  0000000000000000(0000) GS:ffff8882e2724000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000051 CR3: 00000001110a5003 CR4: 00000000003706f0
> Call Trace:
>  <TASK>
>  pick_next_task_fair+0x1d/0x3d0
>  __schedule+0x1ee/0x10c0
>  ? lock_timer_base+0x6d/0x90
>  ? rcu_gp_cleanup+0x560/0x560
>  schedule+0x23/0xc0
>  schedule_timeout+0x6e/0xe0
>  ? hrtimers_cpu_dying+0x1b0/0x1b0
>  rcu_gp_fqs_loop+0xfb/0x510
>  rcu_gp_kthread+0xcd/0x160
>  kthread+0xf5/0x1e0
>  ? kthreads_online_cpu+0x100/0x100
>  ? kthreads_online_cpu+0x100/0x100
>  ret_from_fork+0x114/0x140
>  ? kthreads_online_cpu+0x100/0x100
>  ret_from_fork_asm+0x11/0x20
>  </TASK>
> Modules linked in: i915 drm_buddy intel_gtt drm_client_lib
> drm_display_helper drm_kms_helper igb cec dca rc_core i2c_algo_bit ttm
> agpgart e1000e serio_raw hwmon drm mei_wdt i2c_core intel_oc_wdt video wmi
> CR2: 0000000000000051
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:pick_task_fair+0x57/0x160
> Code: 66 90 66 90 48 8b 5d 50 48 85 db 74 10 48 8b 73 70 48 89 ef e8 3a 74
> ff ff 85 c0 75 71 be 01 00 00 00 48 89 ef e8 29 a5 ff ff <80> 78 51 00 48 89
> c3 0f 85 80 00 00 00 48 85 c0 0f 84 87 00 00 00
> RSP: 0000:ffffc900000d3cf8 EFLAGS: 00010086
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000800
> RDX: fffffc02295d3c00 RSI: 0000000000000800 RDI: 0000000002edc4f2
> RBP: ffff888108f13000 R08: 0000000000000400 R09: 0000000000000002
> R10: 0000000000000260 R11: ffff888108b74200 R12: ffff888265c2cd00
> R13: 0000000000000000 R14: ffff888265c2cd80 R15: ffffffff827c6fa0
> FS:  0000000000000000(0000) GS:ffff8882e2724000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000051 CR3: 00000001110a5003 CR4: 00000000003706f0
> Kernel panic - not syncing: Fatal exception
> Shutting down cpus with NMI
> Kernel Offset: disabled
> Rebooting in 30 seconds..
> ---
> 
> I can't seem to reproduce this issue with the v6.18.1 tagged linux-stable
> build, and
> after bisecting between v6.18.1 and v6.18.2, I land on this commit (which is
> clearly
> not the problem):
> 
> d911fa97dab3ba026a8b96bb7f833d007b7fc4e1 | wifi: ath12k: fix VHT MCS
> assignment
> 
> I don't have any ath12k radios in my system, but I do have 1 ath9k and 2
> ath10k
> radios. A little up the tree I see this patch which *could* be related, but
> I lack
> the knowledge to know:
> 
> b1497ea246396962156b63d5c568a16d6e32de0b | wifi: ath10k: move recovery check
> logic into a new work
> 
> Let me know if there's any more info that's needed or additional steps I can
> take to
> further diagnose the bug.

This should be the same issue as reported in
https://lore.kernel.org/oe-lkp/202510211205.1e0f5223-lkp@intel.com/
and then fixed in mainline with 127b90315ca0 ("sched/proxy: Yield the
donor task") .

Can you confirm picking this commit fixes the issue?

Regards,
Salvatore

Re: [BUG 6.18.2] Null Pointer Exception in Fair Scheduler

[Holger Hoffstätte]

On 2025-12-31 21:00, Dylan E. wrote:
> Hello,
> 
> When booting into the v6.18.2 tagged kernel from linux-stable, I get the following
> stack trace while booting into the system every 1 in 5 boots or so, usually during
> fsck or early systemd service initialization:
> 
> ---
> BUG: kernel NULL pointer dereference, address: 0000000000000051
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> Oops: Oops: 0000 [#1] SMP
> CPU: 0 UID: 0 PID: 15 Comm: rcu_preempt Not tainted 6.18.2 #2 PREEMPT(full)
> Hardware name:  /SKYBAY, BIOS 5.12 06/27/2017
> RIP: 0010:pick_task_fair+0x57/0x160
> Code: 66 90 66 90 48 8b 5d 50 48 85 db 74 10 48 8b 73 70 48 89 ef e8 3a 74 ff ff 85 c0 75 71 be 01 00 00 00 48 89 ef e8 29 a5 ff ff <80> 78 51 00 48 89 c3 0f 85 80 00 00 00 48 85 c0 0f 84 87 00 00 00
> RSP: 0000:ffffc900000d3cf8 EFLAGS: 00010086
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000800
> RDX: fffffc02295d3c00 RSI: 0000000000000800 RDI: 0000000002edc4f2
> RBP: ffff888108f13000 R08: 0000000000000400 R09: 0000000000000002
> R10: 0000000000000260 R11: ffff888108b74200 R12: ffff888265c2cd00
> R13: 0000000000000000 R14: ffff888265c2cd80 R15: ffffffff827c6fa0
> FS:  0000000000000000(0000) GS:ffff8882e2724000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000051 CR3: 00000001110a5003 CR4: 00000000003706f0
> Call Trace:
>   <TASK>
>   pick_next_task_fair+0x1d/0x3d0
>   __schedule+0x1ee/0x10c0

Welcome to the club :) I already reported it and requested a fix in:
https://lore.kernel.org/stable/04b82346-c38a-08e2-49d5-d64981eb7dae@applied-asynchrony.com/

You can apply the patch from:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=127b90315ca07ccad2618db7ba950a63e3b32d22
which should fix it.

cheers
Holger

Re: [BUG 6.18.2] Null Pointer Exception in Fair Scheduler

[Greg KH]

On Wed, Dec 31, 2025 at 09:59:51PM +0100, Holger Hoffstätte wrote:
> On 2025-12-31 21:00, Dylan E. wrote:
> > Hello,
> > 
> > When booting into the v6.18.2 tagged kernel from linux-stable, I get the following
> > stack trace while booting into the system every 1 in 5 boots or so, usually during
> > fsck or early systemd service initialization:
> > 
> > ---
> > BUG: kernel NULL pointer dereference, address: 0000000000000051
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > PGD 0 P4D 0
> > Oops: Oops: 0000 [#1] SMP
> > CPU: 0 UID: 0 PID: 15 Comm: rcu_preempt Not tainted 6.18.2 #2 PREEMPT(full)
> > Hardware name:  /SKYBAY, BIOS 5.12 06/27/2017
> > RIP: 0010:pick_task_fair+0x57/0x160
> > Code: 66 90 66 90 48 8b 5d 50 48 85 db 74 10 48 8b 73 70 48 89 ef e8 3a 74 ff ff 85 c0 75 71 be 01 00 00 00 48 89 ef e8 29 a5 ff ff <80> 78 51 00 48 89 c3 0f 85 80 00 00 00 48 85 c0 0f 84 87 00 00 00
> > RSP: 0000:ffffc900000d3cf8 EFLAGS: 00010086
> > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000800
> > RDX: fffffc02295d3c00 RSI: 0000000000000800 RDI: 0000000002edc4f2
> > RBP: ffff888108f13000 R08: 0000000000000400 R09: 0000000000000002
> > R10: 0000000000000260 R11: ffff888108b74200 R12: ffff888265c2cd00
> > R13: 0000000000000000 R14: ffff888265c2cd80 R15: ffffffff827c6fa0
> > FS:  0000000000000000(0000) GS:ffff8882e2724000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000000000000051 CR3: 00000001110a5003 CR4: 00000000003706f0
> > Call Trace:
> >   <TASK>
> >   pick_next_task_fair+0x1d/0x3d0
> >   __schedule+0x1ee/0x10c0
> 
> Welcome to the club :) I already reported it and requested a fix in:
> https://lore.kernel.org/stable/04b82346-c38a-08e2-49d5-d64981eb7dae@applied-asynchrony.com/
> 
> You can apply the patch from:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=127b90315ca07ccad2618db7ba950a63e3b32d22
> which should fix it.

Will be in the next 6.18.y release, thanks.

greg k-h

Re: [BUG 6.18.2] Null Pointer Exception in Fair Scheduler

[Dylan E.]

Hello again,

On 6.18.3, I'm still seeing the scheduler NPE pop-up, though this one 
happened at shutdown. Looks to be an identical location to the previous 
stack trace.

---
BUG: kernel NULL pointer dereference, address: 0000000000000051
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP
CPU: 0 UID: 0 PID: 336 Comm: (udev-worker) Not tainted 6.18.3 #39 
PREEMPT(full)
Hardware name: Default string Default string/SKYBAY, BIOS 5.12 02/21/2023
RIP: 0010:pick_task_fair+0x57/0x160
Code: 66 90 66 90 48 8b 5d 50 48 85 db 74 10 48 8b 73 70 48 89 ef e8 3a 
74 ff ff 85 c0 75 71 be 01 00 00 00 48 89 ef e8 29 a5 ff ff <80> 78 51 
00 48 89 c3 0f 85 80 00 00 00 48 85 c0 0f 84 87 00 00 00
RSP: 0018:ffffc9000091bca0 EFLAGS: 00010082
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000c00
RDX: fffffa1b1b2acc00 RSI: 0000000000001000 RDI: 0000000000000400
RBP: ffff8881255ef000 R08: 0000000000000400 R09: 0000000000000002
R10: 00000000000003ae R11: ffff88812d2cc200 R12: ffff88845dc2cd00
R13: 0000000000000000 R14: ffff88845dc2cd80 R15: ffffffff827c71e0
FS:  00007fed68640980(0000) GS:ffff8884da723000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000051 CR3: 00000001244dd006 CR4: 00000000003706f0
Call Trace:
  <TASK>
  pick_next_task_fair+0x1d/0x3d0
  __schedule+0x1ee/0x10c0
  ? sock_def_readable+0x3e/0xb0
  ? sock_poll+0x4d/0xd0
  schedule+0x23/0xc0
  schedule_hrtimeout_range_clock+0xf4/0x100
  do_epoll_wait+0x481/0x4b0
  ? ep_destroy_wakeup_source+0x20/0x20
  __x64_sys_epoll_wait+0x5a/0xf0
  ? syscall_trace_enter+0x8d/0x1a0
  do_syscall_64+0x50/0x3a0
  entry_SYSCALL_64_after_hwframe+0x4b/0x53
RIP: 0033:0x7fed69033ea7
Code: 0c 00 f7 d8 64 89 02 b8 ff ff ff ff eb be 0f 1f 44 00 00 f3 0f 1e 
fa 80 3d c5 a4 0c 00 00 41 89 ca 74 10 b8 e8 00 00 00 0f 05 <48> 3d 00 
f0 ff ff 77 51 c3 55 48 89 e5 48 83 ec 20 89 55 f8 48 89
RSP: 002b:00007ffd7c3b5508 EFLAGS: 00000202 ORIG_RAX: 00000000000000e8
RAX: ffffffffffffffda RBX: 0000000000000006 RCX: 00007fed69033ea7
RDX: 0000000000000006 RSI: 0000558994c733e0 RDI: 0000000000000003
RBP: 00007ffd7c3b5620 R08: 0000558994c42010 R09: 0000000000000003
R10: 00000000ffffffff R11: 0000000000000202 R12: 0000558994c733e0
R13: 0000000000000002 R14: 0000558994c73060 R15: ffffffffffffffff
  </TASK>
Modules linked in: i915 drm_buddy intel_gtt drm_client_lib 
drm_display_helper drm_kms_helper cec rc_core ttm agpgart ixgbe igb mdio 
i2c_algo_bit libie_fwlog drm dca hwmon intel_oc_wdt i2c_core mei_wdt 
video wmi
CR2: 0000000000000051
---[ end trace 0000000000000000 ]---
RIP: 0010:pick_task_fair+0x57/0x160
Code: 66 90 66 90 48 8b 5d 50 48 85 db 74 10 48 8b 73 70 48 89 ef e8 3a 
74 ff ff 85 c0 75 71 be 01 00 00 00 48 89 ef e8 29 a5 ff ff <80> 78 51 
00 48 89 c3 0f 85 80 00 00 00 48 85 c0 0f 84 87 00 00 00
RSP: 0018:ffffc9000091bca0 EFLAGS: 00010082
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000c00
RDX: fffffa1b1b2acc00 RSI: 0000000000001000 RDI: 0000000000000400
RBP: ffff8881255ef000 R08: 0000000000000400 R09: 0000000000000002
R10: 00000000000003ae R11: ffff88812d2cc200 R12: ffff88845dc2cd00
R13: 0000000000000000 R14: ffff88845dc2cd80 R15: ffffffff827c71e0
FS:  00007fed68640980(0000) GS:ffff8884da723000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000051 CR3: 00000001244dd006 CR4: 00000000003706f0
Kernel panic - not syncing: Fatal exception
Shutting down cpus with NMI
Kernel Offset: disabled
Rebooting in 30 seconds..
---

Let me know if I can provide anymore information!

-- Dylan

Re: [BUG 6.18.2] Null Pointer Exception in Fair Scheduler

[Greg KH]

On Tue, Jan 06, 2026 at 10:56:49AM -0800, Dylan E. wrote:
> Hello again,
> 
> On 6.18.3, I'm still seeing the scheduler NPE pop-up, though this one
> happened at shutdown. Looks to be an identical location to the previous
> stack trace.

I don't think the fix landed in 6.18.3, can you try 6.18.4?

thanks,

greg k-h

Re: [BUG 6.18.2] Null Pointer Exception in Fair Scheduler

[Dylan E.]

Hi Greg,

 > I don't think the fix landed in 6.18.3, can you try 6.18.4? At least 
with cursory effort, I can't reproduce with 6.18.4 (or with the 
previously suggested patch applied to 6.18.3). Will update if I do see 
anything though. Thanks! -- Dylan