Re: [PATCH v4 2/3] genirq: reimplement exit_irq_thread() hook via task_work_add()

[David Howells <dhowells@redhat.com>]

Oleg Nesterov <oleg@redhat.com> wrote:

> 2. change irq_thread() to do task_queue_work(irq_thread_dtor)
>    at the start and task_work_cancel() before return.
> 
>    tracehook_notify_resume() can never play with kthreads,
>    only do_exit()->exit_task_work() can call the callback
>    and this is what we want.

Hmmm...  This seems wrong.  You're now using the hook in two distinct ways:
the primary use of that the hook is to detect that userspace is about to
resume processing (via TIF_NOTIFY_RESUME) and then you're abusing the fact
that the hook is also invoked via do_exit() to perform a clean up because
we've got to get rid of it somehow under that circumstance.

This only works for you because you're operating in a kernel thread which
doesn't have a userspace (and so will never see TIF_NOTIFY_RESUME).  However,
if someone tries that in an ordinary thread, it is liable to malfunction as
the record could be executed and deleted at some unpredictable point in the
future.

David