linux-kernel - Re: [PATCH] tracing: Choose static tp_printk buffer by explicit nesting count

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Date:	Wed, 25 May 2016 09:36:04 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] tracing: Choose static tp_printk buffer by explicit
 nesting count

On Wed, 25 May 2016 15:16:40 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> >  static char *get_trace_buf(void)
> >  {
> > +	struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);
> >  
> > +	if (!buffer || buffer->nesting >= 4)
> >  		return NULL;  
> 
> This is buggy fwiw; you need to unconditionally increment
> buffer->nesting to match the unconditional decrement.
> 
> Otherwise 5 'increments' and 5 decrements will land you at -1.

As I said, I did a quick look and haven't reviewed it.

Peter, thanks for looking at it.

> 
> >  
> > +	return &buffer->buffer[buffer->nesting++][0];
> > +}
> > +
> > +static void put_trace_buf(void)
> > +{
> > +	this_cpu_dec(trace_percpu_buffer->nesting);
> >  }  
> 
> So I don't know about tracing; but for perf this construct would not
> work 'properly'.
> 
> The per context counter -- which is lost in this scheme -- guards
> against in-context recursion.
> 
> Only if we nest from another context do we allow generation of a new
> event.

The ring buffer itself has a context check, where if you try to record
another event nested in the same context, it will simply return NULL.

But this buffer is only used for trace_printk() to manipulate a printf
format. It writes into here first, and then copies it into the tracing
ring buffer. If it happens at a nested event within the same context,
then trace_buffer_lock_reserve() will return NULL and the event wont be
recorded.

-- Steve