Re: [RESEND][PATCH v15 0/4] perf: Support the deferred unwinding infrastructure

On Thu, Sep 18, 2025 at 07:24:14PM +0200, Peter Zijlstra wrote:

So we have:

do_syscall_64()
  ... do stuff ...
  syscall_exit_to_user_mode(regs)
    syscall_exit_to_user_mode_work(regs)
      syscall_exit_work()
      exit_to_user_mode_prepare()
        exit_to_user_mode_loop()
	  retume_user_mode_work()
	    task_work_run()
    exit_to_user_mode()
      unwind_reset_info();
      user_enter_irqoff();
      arch_exit_to_user_mode();
      lockdep_hardirqs_on();
  SYSRET/IRET


and

DEFINE_IDTENTRY*()
  irqentry_enter();
  ... stuff ...
  irqentry_exit()
    irqentry_exit_to_user_mode()
      exit_to_user_mode_prepare()
        exit_to_user_mode_loop();
	  retume_user_mode_work()
	    task_work_run()
      exit_to_user_mode()
        unwind_reset_info();
	...
  IRET

Now, task_work_run() is in the exit_to_user_mode_loop() which is notably
*before* exit_to_user_mode() which does the unwind_reset_info().

What happens if we get an NMI requesting an unwind after
unwind_reset_info() while still very much being in the kernel on the way
out?

AFAICT it will try and do a task_work_add(TWA_RESUME) from NMI context,
and this will fail horribly.

If you do something like:

	twa_mode = in_nmi() ? TWA_NMI_CURRENT : TWA_RESUME;
	task_work_add(foo, twa_mode);

it might actually work.