tracing: Make tracepoint lockdep check actually test something

tracing: Make tracepoint lockdep check actually test something

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@redhat.com
Link: http://lore.kernel.org/lkml/20140807175204.C257CAC5@viggo.jf.intel.com
Link: https://lore.kernel.org/lkml/20230307184645.521db5c9@gandalf.local.home/
Link: https://lore.kernel.org/linux-trace-kernel/20230310172856.77406446@gandalf.local.home
Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>