Re: [BUG] Deadlock triggered by bpfsnoop funcgraph feature

[Leon Hwang <hffilwlqm@gmail.com>]

On 29/8/25 01:24, Paul E. McKenney wrote:
> On Thu, Aug 28, 2025 at 09:43:00AM -0700, Alexei Starovoitov wrote:
>> On Thu, Aug 28, 2025 at 6:39 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>>>
>>> On Thu Aug 28, 2025 at 7:50 PM +08, Paul E. McKenney wrote:

[...]

>>>
>>> I think it would be better to add "notrace" to following functions:
>>>
>>> ./bpfsnoop -k 'rcu_read_*lock_*held*,rcu_lockdep_*' --show-func-proto
>>> bool rcu_lockdep_current_cpu_online(); [traceable]
>>> int rcu_read_lock_any_held(); [traceable]
>>> int rcu_read_lock_bh_held(); [traceable]
>>> int rcu_read_lock_held(); [traceable]
>>> int rcu_read_lock_sched_held(); [traceable]
>>
>> Agree. Seems like an easy way to remove a footgun.
> 
> Very good, and please see below.  This might or might not make the next
> merge window, but if not, it should be good for the one after that.
> 
>> Independently it would be good to make noinstr/notrace to include __cpuidle
>> functions. I think right now it's allowed to attach to default_idle()
>> which is causing issues.

Nope.
./bpfsnoop -k 'default_idle' -k '__cpuidle' --show-func-proto
void default_idle();

Here are the traced functions of
./bpfsnoop -D -k "htab_*_elem" --output-fgraph --fgraph-debug
--fgraph-exclude
'rcu_read_lock_*held,rcu_lockdep_current_cpu_online,*raw_spin_*lock*,kvfree,show_stack,put_task_stack':

htab_lru_map_delete_elem
htab_lru_map_lookup_and_delete_elem
htab_lru_map_lookup_elem
htab_lru_map_update_elem
htab_lru_percpu_map_lookup_and_delete_elem
htab_lru_percpu_map_lookup_elem
htab_lru_percpu_map_lookup_percpu_elem
htab_lru_percpu_map_update_elem
htab_map_delete_elem
htab_map_lookup_and_delete_elem
htab_map_lookup_elem
htab_map_seq_show_elem
htab_map_update_elem
htab_of_map_lookup_elem
htab_percpu_map_lookup_and_delete_elem
htab_percpu_map_lookup_elem
htab_percpu_map_lookup_percpu_elem
htab_percpu_map_seq_show_elem
htab_percpu_map_update_elem
add_taint
alloc_htab_elem
arch_scale_cpu_capacity
arch_stack_walk
bpf_list_head_free
__bpf_obj_drop_impl
bpf_obj_free_fields
__bpf_prog_put
bpf_prog_put
bpf_rb_root_free
bpf_timer_cancel_and_free
bpf_wq_cancel_and_free
btf_find_struct_meta
btf_is_kernel
btf_type_seq_show
btf_type_seq_show_flags
btf_type_show
check_and_free_fields
check_panic_on_warn
check_timeout
clear_pending_if_disabled
console_verbose
copy_map_value_locked
ctx_sched_out
__delayacct_blkio_start
__do_set_cpus_allowed
do_trace_write_msr
enter_lazy_tlb
__folio_put
__folio_unqueue_deferred_split
free_frozen_pages
free_htab_elem
free_huge_folio
get_free_pages_noprof
get_state_synchronize_rcu
get_state_synchronize_rcu_full
get_taint
hrtick_start_fair
hrtimer_active
hrtimer_cancel
hrtimer_setup
hrtimer_start_range_ns
__htab_lru_percpu_map_update_elem
__htab_map_lookup_and_delete_elem
__htab_map_lookup_elem
htab_map_update_elem_in_place
invalidate_user_asid
__is_kernel_percpu_address
is_kernel_percpu_address
is_module_address
__is_module_percpu_address
is_module_percpu_address
is_vmalloc_addr
kallsyms_lookup
kexec_crash_loaded
kvfree_call_rcu
local_clock
__mem_cgroup_uncharge
__might_sleep
mm_needs_global_asid
mod_node_page_state
__msecs_to_jiffies
nbcon_cpu_emergency_enter
nbcon_cpu_emergency_exit
nbcon_get_cpu_emergency_nesting
pcpu_copy_value
perf_cgroup_switch
perf_clear_dirty_counters
perf_ctx_disable
perf_ctx_enable
perf_ctx_sched_task_cb
__perf_event_task_sched_out
perf_event_update_time
perf_event_update_userpage
perf_exclude_event
perf_pmu_sched_task
___perf_sw_event
perf_swevent_event
pick_next_task_fair
pick_task_fair
pick_task_fair
pick_task_idle
preempt_model_str
__printk_cpu_sync_put
__printk_cpu_sync_try_get
__printk_cpu_sync_wait
printk_percpu_data_ready
print_modules
print_stop_info
print_tainted
print_tainted_verbose
print_worker_info
profile_hits
put_prev_entity
queue_delayed_work_on
queued_spin_lock_slowpath
__queue_work
queue_work_on
rcu_note_context_switch
rcu_qs
rcu_report_exp_cpu_mult
rcu_tasks_trace_qs_blkd
rcu_trc_cmpxchg_need_qs
resilient_queued_spin_lock_slowpath
sched_balance_newidle
__schedule_bug
__schedule_delayed_monitor_work
seq_putc
seq_write
set_next_entity
stack_trace_print
stack_trace_save
switch_ldt
switch_mm_irqs_off
synchronize_rcu
task_h_load
task_work_add
this_cpu_in_panic
touch_all_softlockup_watchdogs
__traceiter_contention_begin
__traceiter_contention_end
__traceiter_lock_acquire
__traceiter_lock_release
__traceiter_rcu_preempt_task
__traceiter_sched_entry_tp
__traceiter_sched_exit_tp
__traceiter_tlb_flush
unpin_user_page
__update_context_time
update_rq_clock

> 
> Leon, would you be interested in putting together a patch for these?
> 

I think we can apply this patch first.

If other patch is required, I would like to send it later.

Thanks,
Leon