Re: [BUG] perf/core: Task stuck on global_ctx_data_rwsem

[Namhyung Kim]

Added a subject prefix and CC LKML.

Thanks,
Namhyung

On Mon, Dec 22, 2025 at 03:34:23PM -0800, Namhyung Kim wrote:
> 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
>