Subject: Re: [syzbot] [ext4?] INFO: rcu detected stall in unwind_next_frame (5)

From: syzbot <[email-masked]>
Date: Thu, 11 Dec 2025 05:41:24 -0800

syzbot has found a reproducer for the following issue on:

HEAD commit:    5ce74bc1b7cb Add linux-next specific files for 20251211
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10fd91c2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b9f785244b836412
dashboard link: https://syzkaller.appspot.com/bug?extid=0f5d9b52b3f467f78d36
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14a2261a580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16f00592580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/e550c10060d5/disk-5ce74bc1.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/80331ba2b4cc/vmlinux-5ce74bc1.xz
kernel image: https://storage.googleapis.com/syzbot-assets/4bcb4f82dfcf/bzImage-5ce74bc1.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email-masked]

rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: 	Tasks blocked on level-0 rcu_node (CPUs 0-1): P5976/1:b..l
rcu: 	(detected by 1, t=10502 jiffies, g=13301, q=280 ncpus=2)
task:kworker/u8:2    state:R  running task     stack:24320 pid:5976  tgid:5976  ppid:2      task_flags:0x4208060 flags:0x00080000
Workqueue: kvfree_rcu_reclaim kfree_rcu_work
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5256 [inline]
 __schedule+0x14bc/0x5000 kernel/sched/core.c:6863
 preempt_schedule_irq+0xb5/0x150 kernel/sched/core.c:7190
 irqentry_exit+0x5d8/0x660 kernel/entry/common.c:216
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:lock_acquire+0x16c/0x340 kernel/locking/lockdep.c:5872
Code: 00 00 00 00 9c 8f 44 24 30 f7 44 24 30 00 02 00 00 0f 85 cd 00 00 00 f7 44 24 08 00 02 00 00 74 01 fb 65 48 8b 05 74 e0 24 11 <48> 3b 44 24 58 0f 85 e5 00 00 00 48 83 c4 60 5b 41 5c 41 5d 41 5e
RSP: 0018:ffffc90003a772d8 EFLAGS: 00000206
RAX: a754b79da94c6400 RBX: 0000000000000000 RCX: a754b79da94c6400
RDX: 00000000c3648bbc RSI: ffffffff8dc8f224 RDI: ffffffff8be251e0
RBP: ffffffff81747f85 R08: ffffffff81747f85 R09: ffffffff8e341a60
R10: ffffc90003a77498 R11: ffffffff81ade980 R12: 0000000000000002
R13: ffffffff8e341a60 R14: 0000000000000000 R15: 0000000000000246
 rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
 rcu_read_lock include/linux/rcupdate.h:867 [inline]
 class_rcu_constructor include/linux/rcupdate.h:1195 [inline]
 unwind_next_frame+0xc2/0x23d0 arch/x86/kernel/unwind_orc.c:495
 arch_stack_walk+0x11c/0x150 arch/x86/kernel/stacktrace.c:25
 stack_trace_save+0x9c/0xe0 kernel/stacktrace.c:122
 kasan_save_stack mm/kasan/common.c:57 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
 kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
 poison_slab_object mm/kasan/common.c:253 [inline]
 __kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
 kasan_slab_free include/linux/kasan.h:235 [inline]
 slab_free_hook mm/slub.c:2540 [inline]
 slab_free_freelist_hook mm/slub.c:2569 [inline]
 slab_free_bulk mm/slub.c:6703 [inline]
 kmem_cache_free_bulk+0x3fb/0xdb0 mm/slub.c:7390
 kfree_bulk include/linux/slab.h:830 [inline]
 kvfree_rcu_bulk+0xe5/0x1f0 mm/slab_common.c:1523
 kfree_rcu_work+0xed/0x170 mm/slab_common.c:1601
 process_one_work+0x93a/0x15a0 kernel/workqueue.c:3279
 process_scheduled_works kernel/workqueue.c:3362 [inline]
 worker_thread+0x9b0/0xee0 kernel/workqueue.c:3443
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x599/0xb30 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
 </TASK>
rcu: rcu_preempt kthread starved for 10543 jiffies! g13301 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
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:R  running task     stack:27136 pid:16    tgid:16    ppid:2      task_flags:0x208040 flags:0x00080000
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5256 [inline]
 __schedule+0x14bc/0x5000 kernel/sched/core.c:6863
 __schedule_loop kernel/sched/core.c:6945 [inline]
 schedule+0x165/0x360 kernel/sched/core.c:6960
 schedule_timeout+0x12b/0x270 kernel/time/sleep_timeout.c:99
 rcu_gp_fqs_loop+0x301/0x1540 kernel/rcu/tree.c:2083
 rcu_gp_kthread+0x99/0x390 kernel/rcu/tree.c:2285
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x599/0xb30 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: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:pv_native_safe_halt+0x13/0x20 arch/x86/kernel/paravirt.c:82
Code: cc cc cc cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 66 90 0f 00 2d 13 d0 22 00 f3 0f 1e fa fb f4 <e9> c8 ed 02 00 cc cc cc cc cc cc cc cc 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90000197de0 EFLAGS: 000002c6
RAX: 77582282823a1b00 RBX: ffffffff8197d83a RCX: 77582282823a1b00
RDX: 0000000000000001 RSI: ffffffff8daa54ce RDI: ffffffff8be251e0
RBP: ffffc90000197f10 R08: ffff8880b87336db R09: 1ffff110170e66db
R10: dffffc0000000000 R11: ffffed10170e66dc R12: ffffffff8fc3cc70
R13: 1ffff11003adcb70 R14: 0000000000000001 R15: 0000000000000001
FS:  0000000000000000(0000) GS:ffff888125ae6000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000002200 CR3: 0000000075c16000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 arch_safe_halt arch/x86/include/asm/paravirt.h:107 [inline]
 default_idle+0x13/0x20 arch/x86/kernel/process.c:767
 default_idle_call+0x73/0xb0 kernel/sched/idle.c:122
 cpuidle_idle_call kernel/sched/idle.c:191 [inline]
 do_idle+0x1ea/0x520 kernel/sched/idle.c:332
 cpu_startup_entry+0x44/0x60 kernel/sched/idle.c:430
 start_secondary+0x101/0x110 arch/x86/kernel/smpboot.c:312
 common_startup_64+0x13e/0x147
 </TASK>

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