perf: Fix hang while freeing sigtrap event

In the Linux kernel, the following vulnerability has been resolved:

perf: Fix hang while freeing sigtrap event

Perf can hang while freeing a sigtrap event if a related deferred
signal hadn't managed to be sent before the file got closed:

perf_event_overflow()
task_work_add(perf_pending_task)

fput()
task_work_add(____fput())

task_work_run()
____fput()
perf_release()
perf_event_release_kernel()
_free_event()
perf_pending_task_sync()
task_work_cancel() -> FAILED
rcuwait_wait_event()

Once task_work_run() is running, the list of pending callbacks is
removed from the task_struct and from this point on task_work_cancel()
can't remove any pending and not yet started work items, hence the
task_work_cancel() failure and the hang on rcuwait_wait_event().

Task work could be changed to remove one work at a time, so a work
running on the current task can always cancel a pending one, however
the wait / wake design is still subject to inverted dependencies when
remote targets are involved, as pictured by Oleg:

T1 T2

fd = perf_event_open(pid => T2->pid); fd = perf_event_open(pid => T1->pid);
close(fd) close(fd)
<IRQ> <IRQ>
perf_event_overflow() perf_event_overflow()
task_work_add(perf_pending_task) task_work_add(perf_pending_task)
</IRQ> </IRQ>
fput() fput()
task_work_add(____fput()) task_work_add(____fput())

task_work_run() task_work_run()
____fput() ____fput()
perf_release() perf_release()
perf_event_release_kernel() perf_event_release_kernel()
_free_event() _free_event()
perf_pending_task_sync() perf_pending_task_sync()
rcuwait_wait_event() rcuwait_wait_event()

Therefore the only option left is to acquire the event reference count
upon queueing the perf task work and release it from the task work, just
like it was done before 3a5465418f5f ("perf: Fix event leak upon exec and file release")
but without the leaks it fixed.

Some adjustments are necessary to make it work:

* A child event might dereference its parent upon freeing. Care must be
taken to release the parent last.

* Some places assuming the event doesn't have any reference held and
therefore can be freed right away must instead put the reference and
let the reference counting to its job.