Re: [for-linus][PATCH 5/5] tracing: Make tracepoint lockdep check actually test something

[Joel Fernandes]

On Tue, Mar 14, 2023 at 7:03 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Mar 14, 2023 at 3:03 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> >
> > A while ago where the trace events had the following:
> >
> >    rcu_read_lock_sched_notrace();
> >    rcu_dereference_sched(...);
> >    rcu_read_unlock_sched_notrace();
> >
> > If the tracepoint is enabled, it could trigger RCU issues if called in
> > the wrong place. And this warning was only triggered if lockdep was
> > enabled. If the tracepoint was never enabled with lockdep, the bug would
> > not be caught. To handle this, the above sequence was done when lockdep
> > was enabled regardless if the tracepoint was enabled or not (although the
> > always enabled code really didn't do anything, it would still trigger a
> > warning).
> >
> > But a lot has changed since that lockdep code was added. One is, that
> > sequence no longer triggers any warning. Another is, the tracepoint when
> > enabled doesn't even do that sequence anymore.
>
> I agree with the change but I am confused by the commit message a bit
> due to "Another is, the tracepoint when enabled doesn't even do that
> sequence anymore.".
>
> Whether the tracepoint was enabled or disabled, it is always doing the
> old sequence because we were skipping the tracepoint's static key test
> before running the sequence. Right?
>
> So how was it not doing the old sequence before?

Ah I see, you meant "It was doing a dummy de-ref", not that "it was
_not_ doing anything". ;-)

So it is good then, but perhaps (optionally) call the code as a dummy
RCU deref which was supposed to trigger a warning. ;-)

 - Joel


>
> Other than that,
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
>  - Joel
>
>
> > The main check we care about today is whether RCU is "watching" or not.
> > So if lockdep is enabled, always check if rcu_is_watching() which will
> > trigger a warning if it is not (tracepoints require RCU to be watching).
> >
> > Note, that old sequence did add a bit of overhead when lockdep was enabled,
> > and with the latest kernel updates, would cause the system to slow down
> > enough to trigger kernel "stalled" warnings.
> >
> > Link: http://lore.kernel.org/lkml/20140806181801.GA4605@xxxxxxxxxx
> > Link: http://lore.kernel.org/lkml/20140807175204.C257CAC5@xxxxxxxxxxxxxxxxxx
> > Link: https://lore.kernel.org/lkml/20230307184645.521db5c9@xxxxxxxxxxxxxxxxxx/
> > Link: https://lore.kernel.org/linux-trace-kernel/20230310172856.77406446@xxxxxxxxxxxxxxxxxx
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU")
> > Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> > ---
> >  include/linux/tracepoint.h | 15 ++++++---------
> >  1 file changed, 6 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index fa1004fcf810..2083f2d2f05b 100644
> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -231,12 +231,11 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> >   * not add unwanted padding between the beginning of the section and the
> >   * structure. Force alignment to the same alignment as the section start.
> >   *
> > - * When lockdep is enabled, we make sure to always do the RCU portions of
> > - * the tracepoint code, regardless of whether tracing is on. However,
> > - * don't check if the condition is false, due to interaction with idle
> > - * instrumentation. This lets us find RCU issues triggered with tracepoints
> > - * even when this tracepoint is off. This code has no purpose other than
> > - * poking RCU a bit.
> > + * When lockdep is enabled, we make sure to always test if RCU is
> > + * "watching" regardless if the tracepoint is enabled or not. Tracepoints
> > + * require RCU to be active, and it should always warn at the tracepoint
> > + * site if it is not watching, as it will need to be active when the
> > + * tracepoint is enabled.
> >   */
> >  #define __DECLARE_TRACE(name, proto, args, cond, data_proto)           \
> >         extern int __traceiter_##name(data_proto);                      \
> > @@ -249,9 +248,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> >                                 TP_ARGS(args),                          \
> >                                 TP_CONDITION(cond), 0);                 \
> >                 if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {             \
> > -                       rcu_read_lock_sched_notrace();                  \
> > -                       rcu_dereference_sched(__tracepoint_##name.funcs);\
> > -                       rcu_read_unlock_sched_notrace();                \
> > +                       WARN_ON_ONCE(!rcu_is_watching());               \
> >                 }                                                       \
> >         }                                                               \
> >         __DECLARE_TRACE_RCU(name, PARAMS(proto), PARAMS(args),          \
> > --
> > 2.39.1