[PATCH] perf: avoid assertion for -p option in perf stat -a

[Stephane Eranian <eranian@google.com>]

It was possible to use perf stat in system-wide mode with -a
and still attach to a process (-p) or thread (-t). The issue
is that it is not possible to get a termination signal from
the attached task, thus the command never terminates.
Furthermore, passing -p -a was triggering the assertion in
read_counter().

This patch fixes the issue by aborting perf stat if the
parameters conflict.

Signed-off-by: Stephane Eranian <eranian@google.com>

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index a6b4d44..a3b6a5f 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -573,6 +573,14 @@ int cmd_stat(int argc, const char **argv, const char *prefix __used)
 	else
 		nr_cpus = 1;
 
+	/*
+	 * a system-wide session cannot be delimited via an attachment
+	 * because we cannot get a termination notification.
+	 * It also triggers the assertion in read_counter()
+	 */
+	if (system_wide && (target_tid != -1 || target_pid != -1))
+		usage_with_options(stat_usage, options);
+
 	if (nr_cpus < 1)
 		usage_with_options(stat_usage, options);