Date: Tue, 25 Apr 2017 14:27:27 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com,
        edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
        bobby.prani@gmail.com
Subject: Re: [PATCH v3 tip/core/rcu 05/40] rcu: Semicolon inside RCU_TRACE()
 for rcu.h
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On Tue, Apr 25, 2017 at 05:13:50PM -0400, Steven Rostedt wrote:
> On Wed, 19 Apr 2017 09:58:02 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
> > The current use of "RCU_TRACE(statement);" can cause odd bugs, especially
> > where "statement" is a local-variable declaration, as it can leave a
> > misplaced ";" in the source code.  This commit therefore converts these
> > to "RCU_TRACE(statement;)", which avoids the misplaced ";".
> > 
> > Reported-by: Josh Triplett <josh@joshtriplett.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> >  kernel/rcu/rcu.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > index 0d6ff3e471be..8700a81daf56 100644
> > --- a/kernel/rcu/rcu.h
> > +++ b/kernel/rcu/rcu.h
> > @@ -109,12 +109,12 @@ static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
> >  
> >  	rcu_lock_acquire(&rcu_callback_map);
> >  	if (__is_kfree_rcu_offset(offset)) {
> > -		RCU_TRACE(trace_rcu_invoke_kfree_callback(rn, head, offset));
> > +		RCU_TRACE(trace_rcu_invoke_kfree_callback(rn, head, offset);)
> 
> I have to ask. Why is this a RCU_TRACE() anyway? tracepoints use jump
> labels, and this is basically a nop here when tracing is off.

Because otherwise the build breaks if CONFIG_RCU_TRACE=n.  So it is
not the tracepoints themselves, but their helper functions.  Worse,
the variables declared for them.  Even worse, in Tiny RCU, the fields
in structures that are used only for tracing.

I freely admit that there might be a better way to structure this,
give or take the Tiny RCU structure fields.

							Thanx, Paul

> -- Steve
> 
> 
> >  		kfree((void *)head - offset);
> >  		rcu_lock_release(&rcu_callback_map);
> >  		return true;
> >  	} else {
> > -		RCU_TRACE(trace_rcu_invoke_callback(rn, head));
> > +		RCU_TRACE(trace_rcu_invoke_callback(rn, head);)
> >  		head->func(head);
> >  		rcu_lock_release(&rcu_callback_map);
> >  		return false;
>