Re: [PATCH] selftests: timers: set-timer-lat: fix hang when std out/err are redirected

From: Greg Hackmann
Date: Fri Sep 22 2017 - 11:42:01 EST

On 09/22/2017 07:42 AM, Shuah Khan wrote:
Hi John/Greg,

On 09/21/2017 02:10 PM, Shuah Khan wrote:
do_timer_oneshot() uses select() as a timer with FD_SETSIZE and readfs
is cleared with FD_ZERO without FD_SET.

When stdout and stderr are redirected, the test hangs in select forever.
Fix the problem calling select() with readfds empty and nfds zero. This
is sufficient for using select() for timer.

With this fix "./set-timer-lat > /dev/null 2>&1" no longer hangs.

Signed-off-by: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
tools/testing/selftests/timers/set-timer-lat.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/tools/testing/selftests/timers/set-timer-lat.c b/tools/testing/selftests/timers/set-timer-lat.c
index ab2fe225e051..3c248f0dc0d5 100644
--- a/tools/testing/selftests/timers/set-timer-lat.c
+++ b/tools/testing/selftests/timers/set-timer-lat.c
@@ -229,7 +229,6 @@ int do_timer_oneshot(int clock_id, int flags)
timer_t tm1;
const int interval = 0;
struct timeval timeout;
- fd_set fds;
int err;
err = setup_timer(clock_id, flags, interval, &tm1);
@@ -238,9 +237,8 @@ int do_timer_oneshot(int clock_id, int flags)
memset(&timeout, 0, sizeof(timeout));
timeout.tv_sec = 5;
- FD_ZERO(&fds);
do {
- err = select(FD_SETSIZE, &fds, NULL, NULL, &timeout);
+ err = select(0, NULL, NULL, NULL, &timeout);
} while (err == -1 && errno == EINTR);

I assume you are good with this fix. I plan to apply it to linux-kselftest fixes
today. Please let me know if you have any objections.

-- Shuah

Thanks for looking into this.

Acked-by: Greg Hackmann <ghackmann@xxxxxxxxxx>