Re: [PATCH RFC tip/core/rcu 11/15] tracepoint: Remove smp_read_barrier_depends() from comment

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Oct 10, 2017 at 01:12:38AM +0000, Mathieu Desnoyers wrote:
> ----- On Oct 9, 2017, at 8:31 PM, rostedt rostedt@goodmis.org wrote:
> 
> > [ added Mathieu ]
> > 
> > On Mon,  9 Oct 2017 17:22:45 -0700
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > 
> >> The comment in tracepoint_add_func() mentions smp_read_barrier_depends(),
> >> whose use should be quite restricted.  This commit updates the comment
> >> to instead mention the smp_store_release() and rcu_dereference_sched()
> >> that the current code actually uses.
> >> 
> >> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >> Cc: Ingo Molnar <mingo@kernel.org>
> >> Cc: Steven Rostedt <rostedt@goodmis.org>
> > 
> > Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > 
> 
> Yeah I think we don't need to spell out the implementation of
> rcu_dereference_sched() there (which actually uses lockless_dereference(),
> which indeed ends up relying on smp_read_barrier_depends()). The comment
> was a bit too low-level for the benefit of its audience.
> 
> I agree with the change.
> 
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Applied, thank you both!

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > -- Steve
> > 
> >> ---
> >>  kernel/tracepoint.c | 9 ++++-----
> >>  1 file changed, 4 insertions(+), 5 deletions(-)
> >> 
> >> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> >> index 685c50ae6300..671b13457387 100644
> >> --- a/kernel/tracepoint.c
> >> +++ b/kernel/tracepoint.c
> >> @@ -212,11 +212,10 @@ static int tracepoint_add_func(struct tracepoint *tp,
> >>  	}
> >>  
> >>  	/*
> >> -	 * rcu_assign_pointer has a smp_wmb() which makes sure that the new
> >> -	 * probe callbacks array is consistent before setting a pointer to it.
> >> -	 * This array is referenced by __DO_TRACE from
> >> -	 * include/linux/tracepoints.h. A matching smp_read_barrier_depends()
> >> -	 * is used.
> >> +	 * rcu_assign_pointer has as smp_store_release() which makes sure
> >> +	 * that the new probe callbacks array is consistent before setting
> >> +	 * a pointer to it.  This array is referenced by __DO_TRACE from
> >> +	 * include/linux/tracepoint.h using rcu_dereference_sched().
> >>  	 */
> >>  	rcu_assign_pointer(tp->funcs, tp_funcs);
> > >  	if (!static_key_enabled(&tp->key))
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>