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

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Date:   Fri, 22 Sep 2017 08:42:14 -0600
From:   Shuah Khan <shuahkh@....samsung.com>
To:     john.stultz@...aro.org, tglx@...utronix.de, sboyd@...eaurora.org,
        ghackmann@...gle.com, shuah@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Shuah Khan <shuahkh@....samsung.com>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH] selftests: timers: set-timer-lat: fix hang when std
 out/err are redirected

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@....samsung.com>
> ---
>  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);
>  
>  	timer_delete(tm1);
> 

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.

thanks,
-- Shuah