perf lock contention: Reject more than 10ms delays for safety

Delaying kernel operations can be dangerous and the kernel may kill
(non-sleepable) BPF programs running for long in the future.

Limit the max delay to 10ms and update the document about it.

  $ sudo ./perf lock con -abl -J 100000us@cgroup_mutex true
  lock delay is too long: 100000us (> 10ms)

   Usage: perf lock contention [<options>]

      -J, --inject-delay <TIME@FUNC>
                            Inject delays to specific locks

Suggested-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Song Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: namhyung

tools/perf/Documentation/perf-lock.txt

@@ -224,8 +224,12 @@ CONTENTION OPTIONS
 	only with -b/--use-bpf.
 
 	The 'time' is specified in nsec but it can have a unit suffix.  Available
-	units are "ms" and "us".  Note that it will busy-wait after it gets the
-	lock.  Please use it at your own risk.
+	units are "ms", "us" and "ns".  Currently it accepts up to 10ms of delays
+	for safety reasons.
+
+	Note that it will busy-wait after it gets the lock. Delaying locks can
+	have significant consequences including potential kernel crashes.  Please
+	use it at your own risk.
 
 
 SEE ALSO

tools/perf/builtin-lock.c

@@ -2537,6 +2537,11 @@ static bool add_lock_delay(char *spec)
 		return false;
 	}
 
+	if (duration > 10 * 1000 * 1000) {
+		pr_err("lock delay is too long: %s (> 10ms)\n", spec);
+		return false;
+	}
+
 	tmp = realloc(delays, (nr_delays + 1) * sizeof(*delays));
 	if (tmp == NULL) {
 		pr_err("Memory allocation failure\n");