[PATCH v5 0/6] 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.

v4…v5: https://lore.kernel.org/all/20240624152732.1231678-1-bigeasy@xxxxxxxxxxxxx/
   - Add TWA_NMI_CURRENT as notify mode for task_work_add() and use it.
     PeterZ pointed out that the current version is not NMI safe.

v3…v4: https://lore.kernel.org/all/20240322065208.60456-1-bigeasy@xxxxxxxxxxxxx/
   - Rebased on top of Frederic's series
      (https://lore.kernel.org/all/20240621091601.18227-1-frederic@xxxxxxxxxx)
   - Frederick pointed out that perf_pending_task() needs to
     perf_swevent_get_recursion_context() in order not to recurse if
     something within perf_swevent_.*_recursion_context() triggers a
     software event. To address this, the counters have been moved to
     the task_struct (#3 + #4) and preemt_disable() has been replaced
     with a RCU-read lock (#5).
   - The remaning logic same that means the event is pushed to task-work
     instead of delivering from IRQ-work. The series was tested with
     remove_on_exec as suggested by Marco Elver: On PREEMPT_RT a single
     invocation passes, 100 parallel invocations report (for some)
     unexpected SIGTRAPs and timeouts. This also observed on !RT
     (without the series) with a higher task-count.

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