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

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Message-ID: <20160525175947.GS3205@twins.programming.kicks-ass.net>
Date:	Wed, 25 May 2016 19:59:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] tracing: Choose static tp_printk buffer by explicit
 nesting count

On Wed, May 25, 2016 at 03:16:40PM +0200, Peter Zijlstra wrote:
> On Tue, May 24, 2016 at 03:52:28PM -0700, Andy Lutomirski wrote:
> > +	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.

Urgh, never mind me; if you don't dec when returning NULL  this should
work out fine.

My head really aint working right today.