On Tue, Mar 14, 2023 at 03:02:41PM -0400, Steven Rostedt 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. > > 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> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > --- > 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
