On 1/3/2026 11:42 PM, Joel Fernandes wrote:
> On 1/3/2026 10:38 PM, 王志 wrote:
>> Dear Developers,
>> I am reporting an RCU CPU stall detected during fuzzing with Syzkaller on
>> Linux 6.18.0-rc.
>>
>> Technical Summary: The stall occurs on CPU 0 while executing a clone
>> syscall. The task was interrupted by a timer softirq which then invoked
>> mac80211_hwsim_beacon. The trace shows it is stuck waiting for a spinlock in
>> mac80211_hwsim_tx_frame_no_nl.
>>
>> Key Logs:
>> RCU GP Starvation: 10509 jiffies
>> Call Trace: native_queued_spin_lock_slowpath <-
>> mac80211_hwsim_tx_frame_no_nl <- hrtimer_run_softirq
>> Environment:
> [...]
>> native_queued_spin_lock_slowpath+0xae/0x9c0
>> debug_spin_lock_before
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/locking/spinlock_debug.c:87
>> [inline]
>> do_raw_spin_lock+0x20d/0x2b0
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/locking/spinlock_debug.c:115
>> mac80211_hwsim_tx_frame_no_nl.isra.0+0x6f1/0x12c0
>
> This seems to be a case of softirq context acquiring a lock without
> spin_lock_bh. That explains the stall.
>
> A quick search on the mailing list showed this was fixed recently:
> https://lore.kernel.org/all/20251128151537.0b4006217137.I688f8b19346e94c1f8de0cdadde072054d4b861c@changeid/
>
> Can you try the patch and see if it fixes? Also CC'ing the author of the
> patch.
To clarify, my assumption was softirq context is sharing code acquiring a lock
with regular process context, that causes a context-recursion deadlock if BH is
not being disabled in this path.
- Joel
>
> - Joel
>
>
>
>> mac80211_hwsim_tx_frame+0x1ed/0x2a0
>> mac80211_hwsim_beacon_tx+0x56d/0x9e0
>> __iterate_interfaces+0x2e0/0x650
>> home/wmy/Fuzzer/third_tool/linux-6.18/net/mac80211/util.c:761
>> ieee80211_iterate_active_interfaces_atomic+0x71/0x1b0
>> home/wmy/Fuzzer/third_tool/linux-6.18/net/mac80211/util.c:797
>> mac80211_hwsim_beacon+0x105/0x1b0
>> __run_hrtimer
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/hrtimer.c:1779 [inline]
>> __hrtimer_run_queues+0x1f5/0xb30
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/hrtimer.c:1841
>> __hrtimer_get_next_event
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/hrtimer.c:578 [inline]
>> hrtimer_update_softirq_timer
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/hrtimer.c:1193 [inline]
>> hrtimer_run_softirq+0x17f/0x350
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/hrtimer.c:1861
>> _test_bit
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/asm-generic/bitops/instrumented-non-atomic.h:141
>> [inline]
>> cpumask_test_cpu
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/cpumask.h:646 [inline]
>> cpu_online
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/cpumask.h:1205 [inline]
>> trace_softirq_exit
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/trace/events/irq.h:142 [inline]
>> handle_softirqs+0x1d4/0x870
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/softirq.c:623
>> __irq_exit_rcu+0x109/0x170
>> home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/include/asm/bitops.h:202
>> irq_exit_rcu+0x9/0x30
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/softirq.c:738
>> sysvec_apic_timer_interrupt+0xa8/0xc0
>> home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/kernel/apic/apic.c:2145
>> </IRQ>
>> <TASK>
>> asm_sysvec_apic_timer_interrupt+0x1a/0x20
>> home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/include/asm/idtentry.h:697
>> RIP: 0010:__sanitizer_cov_trace_pc+0x4b/0x80
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/kcov.c:217
>> Code: 74 1d a9 00 00 0f 00 75 16 a9 00 00 f0 00 74 05 c3 cc cc cc cc 8b 82
>> 7c 16 00 00 85 c0 75 05 e9 5b dc 89 09 8b 82 58 16 00 00 <83> f8 02 75 f0 48
>> 8b 8a 60 16 00 00 8b 92 5c 16 00 00 48 8b 01 48
>> RSP: 0018:ffffc90002d47698 EFLAGS: 00000246
>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff82075ed9
>> RDX: ffff88802a758000 RSI: ffffffff82075ee7 RDI: 0000000000000007
>> RBP: dffffc0000000000 R08: 0000000000000001 R09: fffff940002a92c6
>> R10: 0000000000000000 R11: 0000000000000000 R12: ffffea0001549630
>> R13: ffffea0001549600 R14: ffffea0001549618 R15: ffffea0001549600
>> copy_pte_range home/wmy/Fuzzer/third_tool/linux-6.18/mm/memory.c:1284
>> [inline]
>> copy_pmd_range home/wmy/Fuzzer/third_tool/linux-6.18/mm/memory.c:1390
>> [inline]
>> copy_pud_range home/wmy/Fuzzer/third_tool/linux-6.18/mm/memory.c:1427
>> [inline]
>> copy_p4d_range home/wmy/Fuzzer/third_tool/linux-6.18/mm/memory.c:1451
>> [inline]
>> copy_page_range+0x1c67/0x63a0
>> home/wmy/Fuzzer/third_tool/linux-6.18/mm/memory.c:1539
>> rwsem_assert_held_write
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/rwsem.h:215 [inline]
>> mmap_assert_write_locked
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/mmap_lock.h:76 [inline]
>> vma_end_write_all
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/mmap_lock.h:347 [inline]
>> mmap_write_unlock
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/mmap_lock.h:354 [inline]
>> dup_mmap+0xd9e/0x20d0 home/wmy/Fuzzer/third_tool/linux-6.18/mm/mmap.c:1872
>> futex_init_task
>> home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/futex.h:73 [inline]
>> copy_process+0x36ba/0x76c0
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/fork.c:2230
>> kernel_clone+0xea/0x8b0
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/fork.c:2609
>> __do_sys_clone+0xce/0x120
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/fork.c:2750
>> do_syscall_64+0xcb/0xfa0
>> home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/entry/syscall_64.c:99
>> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>> RIP: 0033:0x7f00649a6d07
>> Code: 00 00 90 f3 0f 1e fa 64 48 8b 04 25 10 00 00 00 45 31 c0 31 d2 31 f6
>> bf 11 00 20 01 4c 8d 90 d0 02 00 00 b8 38 00 00 00 0f 05 <48> 3d 00 f0 ff ff
>> 77 41 41 89 c0 85 c0 75 2c 64 48 8b 04 25 10 00
>> RSP: 002b:00007ffe2d0a2b98 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
>> RAX: ffffffffffffffda RBX: 00007f006575d680 RCX: 00007f00649a6d07
>> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
>> RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000001
>> R10: 0000555572e6d7d0 R11: 0000000000000246 R12: 0000000000000000
>> R13: 0000000000000032 R14: 00007ffe2d0a2d80 R15: 00000000000e717e
>> </TASK>
>> rcu: rcu_preempt kthread starved for 10509 jiffies! g98817 f0x0
>> RCU_GP_WAIT_FQS(5) ->state=0x0 ->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:R running task stack:28520 pid:16 tgid:16
>> ppid:2 task_flags:0x208040 flags:0x00080000
>> Call Trace:
>> <TASK>
>> sched_info_arrive
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/stats.h:267 [inline]
>> sched_info_switch
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/stats.h:330 [inline]
>> prepare_task_switch
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:5122 [inline]
>> context_switch
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:5272 [inline]
>> __schedule+0x1044/0x5bb0
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:6929
>> __schedule_loop
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:7011 [inline]
>> schedule+0xe7/0x3a0
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:7026
>> schedule_timeout+0x113/0x280
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/sleep_timeout.c:98
>> rcu_gp_fqs_check_wake
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/rcu/tree.c:2007 [inline]
>> rcu_gp_fqs_loop+0x18c/0xa00
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/rcu/tree.c:2083
>> rcu_gp_kthread+0x26f/0x370
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/rcu/tree.c:2280
>> kthread+0x3d0/0x780
>> home/wmy/Fuzzer/third_tool/linux-6.18/kernel/kthread.c:463
>> ret_from_fork+0x676/0x7d0
>> home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/kernel/process.c:195
>> ret_from_fork_asm+0x1a/0x30
>> home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/entry/entry_64.S:245
>> </TASK>
>>
>> Best regards,
>> Zhi Wang
>>
>>
>
