checkpatch: Complain about unexpected uses of RCU Tasks Trace

RCU Tasks Trace is quite specialized, having been created specifically
for sleepable BPF programs.  Because it allows general blocking within
readers, any new use of RCU Tasks Trace must take current use cases into
account.  Therefore, update checkpatch.pl to complain about use of any of
the RCU Tasks Trace API members outside of BPF and outside of RCU itself.

[ paulmck: Apply Joe Perches feedback. ]

Cc: Andy Whitcroft <apw@canonical.com> (maintainer:CHECKPATCH)
Cc: Joe Perches <joe@perches.com> (maintainer:CHECKPATCH)
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com> (reviewer:CHECKPATCH)
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

scripts/checkpatch.pl

@@ -7457,6 +7457,30 @@ sub process {
 			}
 		}
 
+# Complain about RCU Tasks Trace used outside of BPF (and of course, RCU).
+		our $rcu_trace_funcs = qr{(?x:
+			rcu_read_lock_trace |
+			rcu_read_lock_trace_held |
+			rcu_read_unlock_trace |
+			call_rcu_tasks_trace |
+			synchronize_rcu_tasks_trace |
+			rcu_barrier_tasks_trace |
+			rcu_request_urgent_qs_task
+		)};
+		our $rcu_trace_paths = qr{(?x:
+			kernel/bpf/ |
+			include/linux/bpf |
+			net/bpf/ |
+			kernel/rcu/ |
+			include/linux/rcu
+		)};
+		if ($line =~ /\b($rcu_trace_funcs)\s*\(/) {
+			if ($realfile !~ m{^$rcu_trace_paths}) {
+				WARN("RCU_TASKS_TRACE",
+				     "use of RCU tasks trace is incorrect outside BPF or core RCU code\n" . $herecurr);
+			}
+		}
+
 # check for lockdep_set_novalidate_class
 		if ($line =~ /^.\s*lockdep_set_novalidate_class\s*\(/ ||
 		    $line =~ /__lockdep_no_validate__\s*\)/ ) {