Re: [PATCH 3/5] ftrace: add ftrace_bprintk()

[Steven Rostedt]

On Mon, 2 Mar 2009, Fr?d?ric Weisbecker wrote:

> 2009/3/2 Steven Rostedt <rostedt@xxxxxxxxxxx>:
> >
> > On Mon, 2 Mar 2009, Fr?d?ric Weisbecker wrote:
> >> >> +
> >> >> +static
> >> >> +void release_module_trace_bprintk_format(const char **start, const char **end)
> >> >> +{
> >> >> +     const char **iter;
> >> >> +     lock_btrace();
> >> >> +     for (iter = start; iter < end; iter++) {
> >> >> +             struct trace_bprintk_fmt *tb_fmt;
> >> >> +             if (!*iter)
> >> >> +                     continue;
> >> >> +
> >> >> +             tb_fmt = container_of(*iter, struct trace_bprintk_fmt, fmt[0]);
> >> >> +             tb_fmt->count--;
> >> >> +             if (!tb_fmt->count && !btrace_metadata_count) {
> >> >> +                     list_del(&tb_fmt->list);
> >> >> +                     kfree(tb_fmt);
> >> >
> >> > Shouldn't *iter get assigned to NULL somewhere here?
> >> >
> >> > -- Steve
> >>
> >>
> >> Hm, why?
> >
> > Well, after we free tb_fmt, the *iter will then point to garbage. Right?
> >
> > -- Steve
> 
> 
> Now that you say it, I have some doubts about the possible sites that
> can still dereference it
> at this point.
> I have to review and test it more seriously. I was convinced that the
> count field kept track
> of all references but now I'm not so sure, there can be still one
> pending event that uses it into
> the ring buffer, or it can be perhaps in use at the same time it is freed.
> We should perhaps use rcu here, will see.
> 

How do you deal with ref counters in the ring buffer? If the ring buffer 
is set to overwrite mode (in which is usually is), then you will never 
know if a print was erased.

I haven't looked too deep into the implementation. But one safe way to
do this, with respect to modules, is the following:

#define ftrace_bprintk(fmt, args...) \
	do { \
		static const char __attribute__((section(ftrace_fmt))\
			*f = fmt; \
		_ftrace_bprintk(&f, args); \
	} while(0)

On output, you can do:

	trace_print_bprintk(...)
	{
		char **f = field->f;

		if (!f)
			trace_seq_printf(s, "MODULE UNLOADED?\n");
		trace_seq_printf(s, *f, ...)

Do you see what I'm doing?

Make the ftrace_printk create a constant pointer to the format instead
of passing in the format. It will istead pass in the address of something
pointing to the format.

Then on module load, you allocate the area and copy in all the ftrace_fmt 
sections.

On module unload, you just NULL out that area. You could probably reuse 
those NULL spots, but you would need some kind of checksum to be added 
such that a new module will be detected on print out.


This is the reason I avoided doing ftrace printk via pointers :-/

-- Steve