Re: [Bug runtime/30831] Systemtap scripts fail to compile on newest linux 6.6 kernels

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2023/09/28 5:03, wcohen at redhat dot com via Systemtap wrote:
> commit ca71442b93af61cbd9a1386e24e967373f928eae (HEAD -> master, origin/master,
> origin/HEAD)
> Author: William Cohen <wcohen@redhat.com>
> Date:   Wed Sep 27 10:09:11 2023 -0400
> 
>     Eliminate use of kernel's flush_scheduled_work() in systemtap modules
> 
>     Kernel git commit 20bdedafd2f63e0ba70991127f9b5c0826ebdb32 turns use
>     of flush_scheduled_work() into a warning which causes builds of
>     anything using it to fail because warnings are treated as errors.
>     Previous users of flush_scheduled_work() in the kernel have been
>     converted over to use individual workqueues.  Systemtap runtime now
>     does the same.  It creates and uses its own workqueue to eliminate the
>     use of flush_scheduled_work().

That commit does not look correct.

  systemtap_wq = alloc_workqueue("systemtap-wq", WQ_UNBOUND | WQ_MEM_RECLAIM, 0);
  if (!systemtap_wq)
    return !systemtap_wq;

causes stap_init_module() to return 1 (i.e. a positive value) when alloc_workqueue()
failed. The caller ( https://elixir.bootlin.com/linux/v6.6/source/kernel/module/main.c#L2528 )
emits a warning message but thinks that stap_init_module() succeeded.
stap_init_module() should return -ENOMEM when alloc_workqueue() failed.

Also, when alloc_workqueue() with WQ_MEM_RECLAIM flag succeeded, alloc_workqueue()
allocated a "struct task_struct". Not calling destroy_workqueue(systemtap_wq) leaks
that "struct task_struct" and related kernel resources.

 rc = systemtap_kernel_module_init();
 if (rc)
   return rc;

I don't know the content of systemtap_kernel_module_init() emitted by translate.cxx ,
but it seems to me that destroy_workqueue(systemtap_wq) will not be called.
Like https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp
says, please be sure to call destroy_workqueue(systemtap_wq) when stap_init_module()
returns an error after alloc_workqueue() succeeded. 

Also, system_wq is created without WQ_MEM_RECLAIM flag
( https://elixir.bootlin.com/linux/v6.6/source/kernel/workqueue.c#L6596 ).
If you don't have a reason to create systemtap_wq with WQ_MEM_RECLAIM flag,
use of WQ_MEM_RECLAIM flag is a waste of kernel resource.