linux-kernel - Re: [PATCH] rcu: define tracepoint strings only if CONFIG

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Date:	Fri, 11 Jul 2014 11:30:10 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-kernel@...r.kernel.org, dipankar@...ibm.com
Subject: Re: [PATCH] rcu: define tracepoint strings only if CONFIG_TRACING
 is set

On Fri, 11 Jul 2014 08:12:40 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Fri, Jul 11, 2014 at 01:38:12PM +0200, Ard Biesheuvel wrote:
> > Commit f7f7bac9cb1c ("rcu: Have the RCU tracepoints use the tracepoint_string
> > infrastructure") unconditionally populates the __tracepoint_str input section,
> > but this section is not assigned an output section if CONFIG_TRACING is not set.
> > This results in the __tracepoint_str turning up in unexpected places, i.e.,
> > after _edata.
> > 
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> > Cc: rostedt@...dmis.org
> > Cc: dipankar@...ibm.com
> > Cc: paulmck@...ux.vnet.ibm.com
> 
> If you get a Reviewed-by from Steven Rostedt, I will be happy to queue
> this one.

I'm fine with it, but it should add a comment.


> 
> 							Thanx, Paul
> 
> > ---
> >  kernel/rcu/tree.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index f1ba77363fbb..ac1984149eb5 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -79,9 +79,16 @@ static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
> >   * the tracing userspace tools to be able to decipher the string
> >   * address to the matching string.
> >   */
> > +#ifdef CONFIG_TRACING

/*
 * When tracing is enabled, DEFINE_TPS() will export the string to
 * userspace via the tracing debugfs directory. This allows userspace
 * tools to read the binary tracepoints that reference the pointer
 * to the string and not the string itself.
 */


> > +#define DEFINE_TPS(sname) \
> > +static const char *tp_##sname##_varname __used __tracepoint_string = sname##_varname;
> > +#else
> > +#define DEFINE_TPS(sname)
> > +#endif

Also, perhaps we should call it:

 DEFINE_RCU_TPS()

Other than that, I'm fine with the patch.

I'll review v2 ;-)

-- Steve

> > +
> >  #define RCU_STATE_INITIALIZER(sname, sabbr, cr) \
> >  static char sname##_varname[] = #sname; \
> > -static const char *tp_##sname##_varname __used __tracepoint_string = sname##_varname; \
> > +DEFINE_TPS(sname) \
> >  struct rcu_state sname##_state = { \
> >  	.level = { &sname##_state.node[0] }, \
> >  	.call = cr, \
> > -- 
> > 1.8.3.2
> > 

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