linux-kernel - Re: [PATCH v3 tip/core/rcu 05/40] rcu: Semicolon inside RCU

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Message-Id: <20170425212727.GH3956@linux.vnet.ibm.com>
Date:   Tue, 25 Apr 2017 14:27:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 05/40] rcu: Semicolon inside RCU_TRACE()
 for rcu.h

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@...ux.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@...htriplett.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.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;
>