[BUG] Task stuck on global_ctx_data_rwsem

[Namhyung Kim <namhyung@kernel.org>]

Hello,

I got a report that a task is stuck in perf_event_exit_task() waiting
for global_ctx_data_rwsem.  On large systems, it'd have performance
issues when it grabs the lock to iterate all threads in the system to
allocate the context data.  And it'd block task exit path which is
problematic especially under memory pressure.

  perf_event_open
    perf_event_alloc
      attach_perf_ctx_data
        attach_global_ctx_data
          percpu_down_write (global_ctx_data_rwsem)
            for_each_process_thread
              alloc_task_ctx_data
                                               do_exit
                                                 perf_event_exit_task
                                                   percpu_down_read (global_ctx_data_rwsem)

I think attach_global_ctx_data() should skip tasks with PF_EXITING and
it'd be nice if perf_event_exit_task() could release the ctx_data
unconditionally.  But I'm not sure how to synchronize them properly.

Any thoughts?

Thanks,
Namhyung