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 08:41:52 -0700
From:   Greg Hackmann <ghackmann@...gle.com>
To:     Shuah Khan <shuahkh@....samsung.com>, john.stultz@...aro.org,
        tglx@...utronix.de, sboyd@...eaurora.org, shuah@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] selftests: timers: set-timer-lat: fix hang when std
 out/err are redirected

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@....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
> 

Thanks for looking into this.

Acked-by: Greg Hackmann <ghackmann@...gle.com>