Subject: Re: [PATCH 11/11 v2] ftrace: Add recording of functions that caused recursion

From: Steven Rostedt <[email-masked]>
Date: Mon, 2 Nov 2020 12:37:21 -0500

On Mon, 2 Nov 2020 17:41:47 +0100
Petr Mladek <[email-masked]> wrote:

> > +	i = atomic_read(&nr_records);
> > +	smp_mb__after_atomic();
> > +	if (i < 0)
> > +		cmpxchg(&recursed_functions[index].ip, ip, 0);
> > +	else if (i <= index)
> > +		atomic_cmpxchg(&nr_records, i, index + 1);  
> 
> This looks weird. It would shift nr_records past the record added
> in this call. It might skip many slots that were zeroed when clearing.
> Also we do not know if our entry was not zeroed as well.

nr_records always holds the next position to write to.

	index = nr_records;
	recursed_functions[index].ip = ip;
	nr_records++;

Before clearing, we have:

	nr_records = -1;
	smp_mb();
	memset(recursed_functions, 0);
	smp_wmb();
	nr_records = 0;

When we enter this function:

	i = nr_records;
	smp_mb();
	if (i < 0)
		return;

Thus, we just stopped all new updates while clearing the records.

But what about if something is currently updating?

	i = nr_records;
	smp_mb();
	if (i < 0)
		cmpxchg(recursed_functions, ip, 0);

The above shows that if the current updating process notices that the
clearing happens, it will clear the function it added.

	else if (i <= index)
		cmpxchg(nr_records, i, index + 1);

This makes sure that nr_records only grows if it is greater or equal to
zero.

The only race that I see that can happen, is the one in the comment I
showed. And that is after enabling the recursed functions again after
clearing, one CPU could add a function while another CPU that just added
that same function could be just exiting this routine, notice that a
clearing of the array happened, and remove its function (which was the same
as the one just happened). So we get a "zero" in the array. If this
happens, it is likely that that function will recurse again and will be
added later.

-- Steve