Re: [PATCH v2 2/4] ftrace: introduce workqueue_handler_exit tracepoint and rename workqueue_execution to workqueue_handler_entry

[Oleg Nesterov <oleg@redhat.com>]

On 04/15, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > > >  		lock_map_acquire(&lockdep_map);
> > > > +		trace_workqueue_handler_entry(cwq->thread, work);
> > > >  		f(work);
> > > > +		trace_workqueue_handler_exit(cwq->thread, work);
> >
> > This doesn't look right. We must not use "work" after f(work).
> > work->func() can kfree its work.
>
> We can use it as long as we use it as a 'cookie' - i.e. an
> identifier for visualization/statistics, but dont actually
> dereference it, right?

Yes sure.

I do not know whether this matters or not, but just in case...
Of course it is possible that, when trace_workqueue_handler_exit() runs,
this memory was already reused for another work even without kfree.
For example,

	void my_work_func(struct work_struct *work)
	{
		INIT_WORK(work, another_work_func);
		queue_work(another_workqueue, work);
	}

In this case another_workqueue can report trace_workqueue_handler_entry()
before my_work_func() returns.

This means that trace_workqueue_handler_exit() can't use the address
of work as a "key" to find some data recorded by _entry(). Unless this
data lives in cpu_workqueue_struct.

Not sure why I am trying to explain the things which are very obvious
to all ;) Just because I don't really understand what these patches do.

Oleg.