Python's json.tool will output the input json to stdout. Redirect to
/dev/null to avoid blocking on stdout writes.

Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/tests/shell/list.sh | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/perf/tests/shell/list.sh b/tools/perf/tests/shell/list.sh
index 8a868ae64560..76a9846cff22 100755
--- a/tools/perf/tests/shell/list.sh
+++ b/tools/perf/tests/shell/list.sh
@@ -24,8 +24,11 @@ trap trap_cleanup EXIT TERM INT
 
 test_list_json() {
   echo "Json output test"
+  # Generate perf list json output into list_output file.
   perf list -j -o "${list_output}"
-  $PYTHON -m json.tool "${list_output}"
+  # Validate the json using python, redirect the json copy to /dev/null as
+  # otherwise the test may block writing to stdout.
+  $PYTHON -m json.tool "${list_output}" /dev/null
   echo "Json output test [Success]"
 }
 
-- 
2.47.0.163.g1226f6d8fa-goog