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@xxxxxxxxxxxxxxxxxxxxxx/

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