[PATCH v3 0/4] perf: Make SIGTRAP and __perf_pending_irq() work on RT.

[Sebastian Andrzej Siewior]

Hi,

Arnaldo reported that "perf test sigtrap" fails on PREEMPT_RT. Sending
the signal gets delayed until event_sched_out() which then uses
task_work_add() for its delivery. This breaks on PREEMPT_RT because the
signal is delivered with disabled preemption.

While looking at this, I also stumbled upon __perf_pending_irq() which
requires disabled interrupts but this is not the case on PREEMPT_RT.

This series aim to address both issues while not introducing a new issue
at the same time ;)
Any testing is appreciated.

v2…v3: https://lore.kernel.org/all/20240312180814.3373778-1-bigeasy@xxxxxxxxxxxxx/
    - Marco suggested to add a few comments
      - Added a comment to __perf_event_overflow() to explain why irq_work
        is raised in the in_nmi() case.
      - Added a comment to perf_event_exit_event() to explain why the
        pending event is deleted.

v1…v2: https://lore.kernel.org/all/20240308175810.2894694-1-bigeasy@xxxxxxxxxxxxx/
    - Marco pointed me to the testsuite that showed two problems:
      - Delayed task_work from NMI / missing events.
        Fixed by triggering dummy irq_work to enforce an interrupt for
	the exit-to-userland path which checks task_work
      - Increased ref-count on clean up/ during exec.
        Mostly addressed by the former change. There is still a window
	if the NMI occurs during execve(). This is addressed by removing
	the task_work before free_event().
      The testsuite (remove_on_exec) fails sometimes if the event/
      SIGTRAP is sent before the sighandler is installed.

Sebastian