Re: [PATCH + QUESTION] bpf: use cond_resched_tasks_rcu_qs in bpf_fd_array_map_clear() loop

[Rik van Riel]

On Wed, 2026-07-15 at 21:53 -0400, Rik van Riel wrote:
> syzkaller creates a PROG_ARRAY with huge max_entries and triggers
> perf
> tracepoint open close in parallel. The hung task detector reports
> "INFO: task hung in perf_tp_event_init" with event_mutex held waiting
> for synchronize_rcu_tasks().
> 
OK, this is more interesting than it looked at first,
and this patch does not look like the right approach,
but I'm also not sure what other approach would work :(

The actual error being thrown is a hung task:



The syzkaller kernel failure is a tragedy in 3 parts.

First, we have perf_event_open() waiting on the event_mutex:

   syz.7.35551 (pid 31354, state D): perf_event_open 
  -> perf_tp_event_init
   -> perf_trace_init
    -> __mutex_lock
     -> blocked on event_mutex.


Second, we have perf_trace_destroy() indirectly
waiting in synchronize_rcu_tasks(), with the
event_mutex held.

    syz.5.35491 (state:I)  do_exit 
  -> task_work_run 
   -> __fput 
    -> perf_release
     -> perf_event_release_kernel
      -> __free_event
       -> perf_trace_destroy
        -> perf_trace_event_close
         -> reg(TRACE_REG_PERF_CLOSE)
          -> perf_ftrace_event_register
           -> perf_ftrace_function_unregister
            -> unregister_ftrace_function 
             -> ftrace_shutdown
              -> synchronize_rcu_tasks()   <-- BLOCKS HERE, forever,
holding event_mutex

Tasks RCU waits for every task to have gone through
a voluntary reschedule, before the grace period can
be advanced. A preemption does not count.


Third, we have a kworker looping for a very long time:
 
  kworker/0:0 (pid 9, TASK_RUNNING, on CPU0): process_one_work
  -> prog_array_map_clear_deferred
   -> bpf_fd_array_map_clear
    -> __fd_array_map_delete_elem. Looping over a huge PROG_ARRAY
max_entries.

With CONFIG_PREEMPT_LAZY=y this gets preempted,
but since involuntary preemptions do not count
for tasks RCU, that does nothnig to help
ftrace_shutdown() get unstuck.


How do we solve this?

Do we actually need to have cond_resched() points
quiesce the task RCU state when running with lazy
preempt, since those are points where we could
preempt voluntarily?

Do we need to prohibit calling synchronize_rcu_tasks()
while holding a mutex?

Do we need to do something else?

What is the best way forward here?

-- 
All Rights Reversed.