[syzbot] [kernel?] BUG: sleeping function called from invalid context in posix_cpu_timer_wait_running

[syzbot <syzbot+143e0859898751aa8f94@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    2709dd5ae32f Merge tag 'sched-urgent-2026-08-22' of git://..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15c5d549580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a70bec3546180b6
dashboard link: https://syzkaller.appspot.com/bug?extid=143e0859898751aa8f94
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
userspace arch: i386
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13c5d549580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/fbfb2a76c4a3/disk-2709dd5a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3e8e5eeead5a/vmlinux-2709dd5a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/644acb46eb2b/bzImage-2709dd5a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+143e0859898751aa8f94@syzkaller.appspotmail.com

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:623
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 5880, name: syz.0.17
preempt_count: 10000, expected: 0
RCU nest depth: 0, expected: 0
locks held by syz.0.17/5880: 0, last CPU#0
Preemption disabled at:
[<ffffffff818b3c57>] percpu_up_read include/linux/percpu-rwsem.h:116 [inline]
[<ffffffff818b3c57>] cgroup_threadgroup_change_end+0x87/0x210 include/linux/cgroup-defs.h:905
CPU: 0 UID: 0 PID: 5880 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 __might_resched+0x36a/0x4c0 kernel/sched/core.c:9234
 __mutex_lock_common kernel/locking/mutex.c:623 [inline]
 __mutex_lock+0x118/0x1550 kernel/locking/mutex.c:821
 posix_cpu_timer_wait_running+0x1d3/0x330 kernel/time/posix-cpu-timers.c:1218
 timer_wait_running kernel/time/posix-timers.c:868 [inline]
 posix_timer_delete+0x349/0x490 kernel/time/posix-timers.c:1061
 exit_itimers+0x1eb/0x3a0 kernel/time/posix-timers.c:1104
 do_exit+0x489/0x2360 kernel/exit.c:967
 do_group_exit+0x22d/0x2f0 kernel/exit.c:1151
 get_signal+0x121b/0x12c0 kernel/signal.c:3046
 arch_do_signal_or_restart+0xbb/0x860 arch/x86/kernel/signal.c:337
 __exit_to_user_mode_loop kernel/entry/common.c:66 [inline]
 exit_to_user_mode_loop+0x10e/0x770 kernel/entry/common.c:101
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
 syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:336 [inline]
 __do_fast_syscall_32+0x437/0x6a0 arch/x86/entry/syscall_32.c:294
 do_fast_syscall_32+0x33/0x70 arch/x86/entry/syscall_32.c:316
 entry_SYSENTER_compat_after_hwframe+0x84/0x8e
RIP: 0023:0xf70af0ec
Code: 90 85 d2 74 0a 89 ce 81 e6 ff 0f 00 00 89 32 85 c0 74 05 c1 e9 0c 89 08 31 c0 5e 5d c3 90 0f 1f 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 58 b8
RSP: 002b:00000000f68845bc EFLAGS: 00000206 ORIG_RAX: 00000000000000f0
RAX: fffffffffffffe00 RBX: 00000000f74a5020 RCX: 0000000000000080
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000f74a5024
RBP: 0000000000000081 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
 </TASK>
----------------
Code disassembly (best guess):
   0:	90                   	nop
   1:	85 d2                	test   %edx,%edx
   3:	74 0a                	je     0xf
   5:	89 ce                	mov    %ecx,%esi
   7:	81 e6 ff 0f 00 00    	and    $0xfff,%esi
   d:	89 32                	mov    %esi,(%rdx)
   f:	85 c0                	test   %eax,%eax
  11:	74 05                	je     0x18
  13:	c1 e9 0c             	shr    $0xc,%ecx
  16:	89 08                	mov    %ecx,(%rax)
  18:	31 c0                	xor    %eax,%eax
  1a:	5e                   	pop    %rsi
  1b:	5d                   	pop    %rbp
  1c:	c3                   	ret
  1d:	90                   	nop
  1e:	0f 1f 00             	nopl   (%rax)
  21:	51                   	push   %rcx
  22:	52                   	push   %rdx
  23:	55                   	push   %rbp
  24:	89 e5                	mov    %esp,%ebp
  26:	0f 34                	sysenter
  28:	cd 80                	int    $0x80
* 2a:	5d                   	pop    %rbp <-- trapping instruction
  2b:	5a                   	pop    %rdx
  2c:	59                   	pop    %rcx
  2d:	c3                   	ret
  2e:	90                   	nop
  2f:	90                   	nop
  30:	90                   	nop
  31:	90                   	nop
  32:	90                   	nop
  33:	90                   	nop
  34:	90                   	nop
  35:	90                   	nop
  36:	90                   	nop
  37:	90                   	nop
  38:	90                   	nop
  39:	90                   	nop
  3a:	90                   	nop
  3b:	90                   	nop
  3c:	90                   	nop
  3d:	90                   	nop
  3e:	58                   	pop    %rax
  3f:	b8                   	.byte 0xb8


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