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
