INFO: rcu detected stall in tun_chr_write_iter

Status: upstream: reported on 2026/06/19 04:37
Reported-by: syzbot+fa0ea0db5038c986b147@syzkaller.appspotmail.com
First crash: 95d, last: 12d
▶ ▼ Similar bugs (3)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
linux-6.6 INFO: rcu detected stall in tun_chr_write_iter 1 1 81d 81d 0/2 upstream: reported on 2026/07/02 23:03
upstream INFO: rcu detected stall in tun_chr_write_iter net 1 1 725d 725d 0/29 auto-obsoleted due to no activity on 2024/12/25 20:05
android-5-15 BUG: soft lockup in tun_chr_write_iter 1 1 786d 786d 0/2 auto-obsoleted due to no activity on 2024/10/25 22:12

Sample crash report:
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: 	0-....: (48 ticks this GP) idle=8f9/1/0x4000000000000002 softirq=111769/111769 fqs=2125 
	(detected by 1, t=10503 jiffies, g=156065, q=1851)
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 PID: 6288 Comm: syz.4.9324 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:check_preemption_disabled+0x4c/0x110 lib/smp_processor_id.c:55
Code: 62 eb 41 76 f7 c1 ff ff ff 7f 74 1f 65 48 8b 0c 25 28 00 00 00 48 3b 4c 24 08 0f 85 c4 00 00 00 48 83 c4 10 5b 41 5e 41 5f 5d <c3> 48 c7 04 24 00 00 00 00 9c 8f 04 24 f7 04 24 00 02 00 00 74 cc
RSP: 0018:ffffc900000071d8 EFLAGS: 00000086
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 8231b58052918f00
RDX: 0000000000000000 RSI: ffffffff8a2b3e80 RDI: ffffffff8a7a4540
RBP: ffffc900000072f8 R08: dffffc0000000000 R09: 1ffffffff203de18
R10: dffffc0000000000 R11: fffffbfff203de19 R12: ffffffff8c31ef60
R13: 1ffff92000000e48 R14: 0000000000000046 R15: dffffc0000000000
FS:  00007f8582ccd6c0(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000007000 CR3: 000000008a476000 CR4: 00000000003506f0
DR0: 0000200000000300 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000200000000300 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Call Trace:
 <IRQ>
 lockdep_recursion_finish kernel/locking/lockdep.c:436 [inline]
 lock_acquire+0x1ae/0x400 kernel/locking/lockdep.c:5625
 rcu_lock_acquire+0x2a/0x30 include/linux/rcupdate.h:313
 rcu_read_lock include/linux/rcupdate.h:740 [inline]
 __perf_event_output kernel/events/core.c:7810 [inline]
 perf_event_output_forward+0xbc/0x2e0 kernel/events/core.c:7832
 __perf_event_overflow+0x35f/0x550 kernel/events/core.c:9525
 perf_swevent_overflow kernel/events/core.c:9601 [inline]
 perf_swevent_event+0x4b8/0x540 kernel/events/core.c:9639
 perf_tp_event+0x324/0xbe0 kernel/events/core.c:10057
 perf_trace_run_bpf_submit+0xf3/0x1c0 kernel/events/core.c:10031
 perf_trace_preemptirq_template+0x2aa/0x360 include/trace/events/preemptirq.h:14
 trace_irq_enable_rcuidle+0x108/0x180 include/trace/events/preemptirq.h:40
 trace_hardirqs_on+0x57/0x80 kernel/trace/trace_preemptirq.c:44
 asm_sysvec_irq_work+0x16/0x20 arch/x86/include/asm/idtentry.h:713
RIP: 0010:rcu_read_unlock_special+0x7f/0x4a0 kernel/rcu/tree_plugin.h:674
Code: eb 03 48 b8 f1 f1 f1 f1 f8 f2 f2 f2 4a 89 04 2b 42 c7 44 2b 08 f8 f3 f3 f3 65 44 8b 35 f2 0f 9d 7e 41 f7 c6 00 00 f0 00 74 40 <48> c7 44 24 20 0e 36 e0 45 4a c7 04 2b 00 00 00 00 42 c7 44 2b 08
RSP: 0018:ffffc900000079c0 EFLAGS: 00000206
RAX: 8231b58052918f00 RBX: 1ffff92000000f3c RCX: 8231b58052918f00
RDX: dffffc0000000000 RSI: ffffffff8a2b2be0 RDI: ffffffff8a7a4540
RBP: ffffc90000007aa0 R08: ffffffff901ef1bf R09: 1ffffffff203de37
R10: dffffc0000000000 R11: fffffbfff203de38 R12: ffffffff8c323400
R13: dffffc0000000000 R14: 0000000000000246 R15: 0000000000000001
 __rcu_read_unlock+0x78/0xd0 kernel/rcu/tree_plugin.h:422
 rcu_read_unlock include/linux/rcupdate.h:772 [inline]
 hsr_announce+0x29a/0x310 net/hsr/hsr_device.c:410
 call_timer_fn+0x17b/0x540 kernel/time/timer.c:1648
 expire_timers kernel/time/timer.c:1699 [inline]
 __run_timers+0x56c/0x810 kernel/time/timer.c:1970
 run_timer_softirq+0x63/0xf0 kernel/time/timer.c:1983
 handle_softirqs+0x336/0x830 kernel/softirq.c:576
 __do_softirq kernel/softirq.c:610 [inline]
 invoke_softirq kernel/softirq.c:450 [inline]
 __irq_exit_rcu+0x13b/0x230 kernel/softirq.c:659
 irq_exit_rcu+0x5/0x20 kernel/softirq.c:671
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1108 [inline]
 sysvec_apic_timer_interrupt+0xa0/0xc0 arch/x86/kernel/apic/apic.c:1108
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x16/0x20 arch/x86/include/asm/idtentry.h:676
RIP: 0010:finish_lock_switch+0x12f/0x290 kernel/sched/core.c:4804
Code: 83 c7 18 be ff ff ff ff e8 0e 1b 6c 08 85 c0 74 46 4d 85 ff 75 62 0f 1f 44 00 00 48 89 df e8 c8 8a 75 08 e8 f3 df 2a 00 fb 5b <41> 5c 41 5d 41 5e 41 5f 5d c3 48 89 df e8 2f 0e fe ff 43 80 3c 2c
RSP: 0018:ffffc9000417ed98 EFLAGS: 00000286
RAX: 8231b58052918f00 RBX: ffff88805c278000 RCX: 8231b58052918f00
RDX: dffffc0000000000 RSI: ffffffff8a2b2be0 RDI: ffffffff8a7a4540
RBP: 1ffff11017207613 R08: ffffffff901ef0f7 R09: 1ffffffff203de1e
R10: dffffc0000000000 R11: fffffbfff203de1f R12: 1ffff110172075c1
R13: dffffc0000000000 R14: ffff8880b903ae08 R15: 0000000000000000
 finish_task_switch+0x12f/0x650 kernel/sched/core.c:4921
 context_switch kernel/sched/core.c:5052 [inline]
 __schedule+0x11cb/0x4350 kernel/sched/core.c:6395
 preempt_schedule_common+0x82/0xd0 kernel/sched/core.c:6571
 preempt_schedule+0xbc/0xd0 kernel/sched/core.c:6596
 preempt_schedule_thunk+0x16/0x18 arch/x86/entry/thunk_64.S:34
 __raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:161 [inline]
 _raw_spin_unlock_irqrestore+0x10d/0x120 kernel/locking/spinlock.c:194
 rcu_preempt_deferred_qs_irqrestore+0x82d/0xbd0 kernel/rcu/tree_plugin.h:-1
 rcu_read_unlock_special+0x3d9/0x4a0 kernel/rcu/tree_plugin.h:673
 __rcu_read_unlock+0x78/0xd0 kernel/rcu/tree_plugin.h:422
 rcu_read_unlock include/linux/rcupdate.h:772 [inline]
 count_memcg_event_mm+0x31f/0x370 include/linux/memcontrol.h:1082
 handle_mm_fault+0x134/0x4440 mm/memory.c:4863
 do_user_addr_fault+0x494/0xc90 arch/x86/mm/fault.c:1355
 handle_page_fault arch/x86/mm/fault.c:1443 [inline]
 exc_page_fault+0x60/0x100 arch/x86/mm/fault.c:1496
 asm_exc_page_fault+0x22/0x30 arch/x86/include/asm/idtentry.h:606
RIP: 0010:copy_user_enhanced_fast_string+0xe/0x40 arch/x86/lib/copy_user_64.S:206
Code: 89 d1 c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 31 c0 0f 01 ca c3 90 90 90 90 90 90 90 0f 01 cb 83 fa 40 0f 82 70 ff ff ff 89 d1 <f3> a4 31 c0 0f 01 ca c3 90 90 90 90 90 90 90 90 90 90 89 d1 83 f8
RSP: 0018:ffffc9000417f838 EFLAGS: 00050202
RAX: ffffffff83e69b01 RBX: 000000000000cfa4 RCX: 0000000000006664
RDX: 000000000000cfa4 RSI: 0000200000007000 RDI: ffff88807c2a6980
RBP: ffffc9000417f988 R08: ffff88807c2acfe3 R09: 1ffff1100f8559fc
R10: dffffc0000000000 R11: ffffed100f8559fd R12: ffff88807c2a0040
R13: 000000000000cfff R14: 00002000000006c0 R15: 0000000000000000
 copy_user_generic arch/x86/include/asm/uaccess_64.h:37 [inline]
 raw_copy_from_user arch/x86/include/asm/uaccess_64.h:52 [inline]
 copyin lib/iov_iter.c:168 [inline]
 _copy_from_iter+0x322/0x1160 lib/iov_iter.c:777
 copy_from_iter include/linux/uio.h:167 [inline]
 skb_copy_datagram_from_iter+0xf1/0x6c0 net/core/datagram.c:567
 tun_get_user+0xb28/0x3b60 drivers/net/tun.c:1837
 tun_chr_write_iter+0x112/0x1e0 drivers/net/tun.c:2019
 call_write_iter include/linux/fs.h:2179 [inline]
 new_sync_write fs/read_write.c:507 [inline]
 vfs_write+0x748/0xd70 fs/read_write.c:594
 ksys_write+0x153/0x260 fs/read_write.c:647
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7f8584a75159
Code: 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 01 48
RSP: 002b:00007f8582ccd028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f8584cfcfa0 RCX: 00007f8584a75159
RDX: 000000000000cfa4 RSI: 00002000000006c0 RDI: 00000000000000c8
RBP: 00007f8584b0c03b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f8584cfd038 R14: 00007f8584cfcfa0 R15: 00007ffef38b67b8
 </TASK>

Crashes (2):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/09/09 11:54 linux-5.15.y 0996e0926f6b 8387c273 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan-perf INFO: rcu detected stall in tun_chr_write_iter
2026/06/19 04:36 linux-5.15.y dc027a595035 a776b0d0 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-5-15-kasan-perf INFO: rcu detected stall in tun_chr_write_iter
* Struck through repros no longer work on HEAD.