[syzbot] [mm?] INFO: rcu detected stall in schedule_tail (8)

[syzbot <syzbot+42836f91edd58eb82c6a@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    12a0094839d0 Merge tag 'timers-urgent-2026-01-24' of git:/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=105cc05a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f1fac0919970b671
dashboard link: https://syzkaller.appspot.com/bug?extid=42836f91edd58eb82c6a
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14557d22580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15d0105a580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/4ede6f8f9210/disk-12a00948.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/47f57ac656a5/vmlinux-12a00948.xz
kernel image: https://storage.googleapis.com/syzbot-assets/22586c0d39b7/bzImage-12a00948.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+42836f91edd58eb82c6a@syzkaller.appspotmail.com

rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: 	Tasks blocked on level-0 rcu_node (CPUs 0-1): P6967/1:b..l
rcu: 	(detected by 0, t=10503 jiffies, g=14029, q=726 ncpus=2)
task:syz-executor    state:R  running task     stack:29456 pid:6967  tgid:6967  ppid:6192   task_flags:0x400040 flags:0x00080000
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5260 [inline]
 __schedule+0xfe4/0x5e10 kernel/sched/core.c:6867
 preempt_schedule_irq+0x50/0x90 kernel/sched/core.c:7194
 irqentry_exit+0x17b/0x670 kernel/entry/common.c:216
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:ptlock_ptr include/linux/mm.h:3240 [inline]
RIP: 0010:pte_lockptr include/linux/mm.h:3264 [inline]
RIP: 0010:pte_offset_map_rw_nolock+0xbd/0x1c0 mm/pgtable-generic.c:330
Code: 80 3c 10 00 0f 85 bb 00 00 00 48 03 1d 5c 91 92 0b 48 b8 00 00 00 00 00 fc ff df 48 8d 7b 28 48 89 fa 48 c1 ea 03 80 3c 02 00 <0f> 85 c0 00 00 00 4c 89 e2 48 8b 5b 28 48 b8 00 00 00 00 00 fc ff
RSP: 0018:ffffc9000bba7ad8 EFLAGS: 00000246
RAX: dffffc0000000000 RBX: ffffea0001ee7580 RCX: ffffffff825bf480
RDX: 1ffffd40003dceb5 RSI: ffffffff825bf3de RDI: ffffea0001ee75a8
RBP: ffff88807b9d60c8 R08: 0000000000000007 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffffc9000bba7c38
R13: 000000007b9d6067 R14: 0000000000000067 R15: 0000000000000001
 handle_pte_fault mm/memory.c:6261 [inline]
 __handle_mm_fault+0xc0e/0x2b50 mm/memory.c:6414
 handle_mm_fault+0x36d/0xa20 mm/memory.c:6583
 do_user_addr_fault+0x74c/0x12f0 arch/x86/mm/fault.c:1385
 handle_page_fault arch/x86/mm/fault.c:1474 [inline]
 exc_page_fault+0x6f/0xd0 arch/x86/mm/fault.c:1527
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0010:__put_user_4+0xd/0x20 arch/x86/lib/putuser.S:94
Code: 66 89 01 31 c9 0f 01 ca c3 cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 48 89 cb 48 c1 fb 3f 48 09 d9 0f 01 cb <89> 01 31 c9 0f 01 ca e9 07 ba 03 00 0f 1f 80 00 00 00 00 90 90 90
RSP: 0018:ffffc9000bba7e78 EFLAGS: 00050202
RAX: 000000000000001b RBX: 0000000000000000 RCX: 0000555562a197d0
RDX: dffffc0000000000 RSI: ffffffff8dc45476 RDI: ffff888025b2a944
RBP: ffff888025b2aba0 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 1ffffffff1cbde81 R12: 1ffff92001774fd6
R13: ffffc9000bba7f48 R14: 0000000000000000 R15: dffffc0000000000
 schedule_tail+0x9e/0xd0 kernel/sched/core.c:5193
 ret_from_fork+0x79/0xaf0 arch/x86/kernel/process.c:154
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
 </TASK>
rcu: rcu_preempt kthread starved for 10518 jiffies! g14029 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 ->cpu=0
rcu: 	Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
rcu: RCU grace-period kthread stack dump:
task:rcu_preempt     state:I stack:27368 pid:16    tgid:16    ppid:2      task_flags:0x208040 flags:0x00080000
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5260 [inline]
 __schedule+0xfe4/0x5e10 kernel/sched/core.c:6867
 preempt_schedule_irq+0x50/0x90 kernel/sched/core.c:7194
 irqentry_exit+0x17b/0x670 kernel/entry/common.c:216
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:debug_object_assert_init+0x7b/0x300 lib/debugobjects.c:1000
Code: f1 f1 f1 c7 40 04 f1 f1 00 00 c7 40 08 00 00 00 f3 c7 40 0c f3 f3 f3 f3 65 48 8b 05 17 95 f4 0e 48 89 84 24 b8 00 00 00 31 c0 <80> 3d 7e bc c7 0b 00 48 89 7c 24 68 48 c7 44 24 60 00 00 00 00 48
RSP: 0018:ffffc90000157910 EFLAGS: 00000246
RAX: 0000000000000000 RBX: ffffc90000157b58 RCX: 0000000000000000
RDX: ffff88801e290000 RSI: ffffffff8b92b940 RDI: ffffc90000157b58
RBP: ffffc90000157a00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: dffffc0000000000
R13: 0000000000000004 R14: ffff88801e290000 R15: 1ffff9200002af26
 debug_timer_assert_init kernel/time/timer.c:803 [inline]
 debug_assert_init kernel/time/timer.c:848 [inline]
 __mod_timer+0xad/0xca0 kernel/time/timer.c:1025
 add_timer+0x62/0x90 kernel/time/timer.c:1249
 schedule_timeout+0x122/0x280 kernel/time/sleep_timeout.c:98
 rcu_gp_fqs_loop+0x1a9/0xb00 kernel/rcu/tree.c:2083
 rcu_gp_kthread+0x21e/0x320 kernel/rcu/tree.c:2285
 kthread+0x3b3/0x730 kernel/kthread.c:463
 ret_from_fork+0x754/0xaf0 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
 </TASK>
rcu: Stack dump where RCU GP kthread last ran:
CPU: 0 UID: 0 PID: 6972 Comm: syz.8.313 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/13/2026
RIP: 0010:__get_user_nocheck_4+0x8/0x20 arch/x86/lib/getuser.S:142
Code: c3 cc cc cc cc 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 01 cb 0f ae e8 8b 10 <31> c0 0f 01 ca c3 cc cc cc cc 66 66 2e 0f 1f 84 00 00 00 00 00 0f
RSP: 0018:ffffc9000546fad8 EFLAGS: 00040202
RAX: 00007f4f28dcf040 RBX: ffff888021280000 RCX: 1ffff1100425033a
RDX: 0000000046505845 RSI: 00007f4f28dce5c0 RDI: ffff888021281a04
RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 00007f4f28dce5c0
R13: ffffc9000546fb90 R14: 0000000000000000 R15: ffff888021281a00
FS:  00007f4f28dcf6c0(0000) GS:ffff8881245dc000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000555562a197d0 CR3: 0000000028f6b000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 check_xstate_in_sigframe arch/x86/kernel/fpu/signal.c:46 [inline]
 __fpu_restore_sig+0x836/0x12f0 arch/x86/kernel/fpu/signal.c:335
 fpu__restore_sig+0x151/0x190 arch/x86/kernel/fpu/signal.c:480
 restore_sigcontext+0x4cb/0x6a0 arch/x86/kernel/signal_64.c:95
 __do_sys_rt_sigreturn+0x229/0x2c0 arch/x86/kernel/signal_64.c:268
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f4f27f9acb7
Code: 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 <0f> 05 48 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89
RSP: 002b:00007f4f28dcf0e8 EFLAGS: 00000246
RAX: 00000000000000ca RBX: 00007f4f28215fa8 RCX: 00007f4f27f9acb9
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00007f4f28215fa8
RBP: 00007f4f28215fa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f4f28216038 R14: 00007ffd840d3410 R15: 00007ffd840d34f8
 </TASK>


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