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From: Philippe Gerum <rpm@xenomai.org>
Message-ID: <c06ad595-f04a-4e0b-5d79-56ae35671965@xenomai.org>
Date: Wed, 28 Mar 2018 11:46:21 +0200
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Subject: Re: [Xenomai] Problem with rt_cond_wait and rt_cond_signal
 combination
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To: Pintu Kumar <pintu.ping@gmail.com>
Cc: "Xenomai@xenomai.org" <xenomai@xenomai.org>

On 03/28/2018 11:32 AM, Pintu Kumar wrote:
> On Wed, Mar 28, 2018 at 1:42 PM, Philippe Gerum <rpm@xenomai.org> wrote:
>> On 03/28/2018 09:54 AM, Pintu Kumar wrote:
>>> Hi,
>>>
>>> We are facing one issue on Xenomai-3 on x86_64 system.
>>> Kernel: 4.9.51
>>> ipipe version: 4
>>> # /usr/xenomai/sbin/version
>>> Xenomai/cobalt v3.0.6 -- #5956064 (2018-03-20 12:13:33 +0100)
>>>
>>> Its a very simple API level test.
>>> We create a condition variable and wait for the condition inside a task.
>>> Then, we immediately signal the condition from the main thread, and simply
>>> wait for the test to finish.
>>>
>>> But we observed that condition task is never getting released, and the
>>> program hangs.
>>>
>>> This is the output:
>>> --------------------------
>>> # ./condsignal
>>> cond_task --> started
>>> main --> cond signal success: 0
>>> Waiting for task to finish....
>>> ^C
>>>
>>>
>>> This is the code snippet that was used.
>>> -----------------------------------------------------
>>> # cat condsignal.c
>>>
>>> /*
>>>  * 1. Create an condition variable and wait inside a task.
>>>  * 2. Delete the condition variable from main task.
>>>  * 3. Observe that the condition wait is released and task ends normally.
>>>  * 4. Finally main task should exit normally.
>>>  *
>>>  * */
>>> #include <stdio.h>
>>> #include <stdlib.h>
>>> #include <sys/mman.h>
>>> #include <alchemy/task.h>
>>> #include <alchemy/mutex.h>
>>> #include <alchemy/cond.h>
>>> #include <alchemy/task.h>
>>>
>>> static RT_COND cond;
>>> static RT_MUTEX mutex;
>>>
>>> static void cond_task(void *cookie)
>>> {
>>>         int err;
>>>
>>>         printf("%s --> started \n", __func__);
>>>         rt_mutex_acquire(&mutex, TM_INFINITE);
>>>         err = rt_cond_wait(&cond, &mutex, TM_INFINITE);
>>>         rt_mutex_release(&mutex);
>>>         if (err) {
>>>                 printf("rt_cond_wait failed, err = %d", err);
>>>                 return;
>>>         }
>>>         printf("%s --> cond wait done\n", __func__);
>>> }
>>>
>>> int main(int argc, char **argv)
>>> {
>>>         int err;
>>>         RT_TASK task1;
>>>
>>>         mlockall(MCL_CURRENT | MCL_FUTURE);
>>>
>>>         err = rt_mutex_create(&mutex, NULL);
>>>         err = rt_cond_create(&cond, "cond_test");
>>>         err = rt_task_create(&task1, NULL, 0, 99, T_JOINABLE);
>>>         err = rt_task_start(&task1, &cond_task, NULL);
>>>         rt_mutex_acquire(&mutex, TM_INFINITE);
>>>         err = rt_cond_signal(&cond);
>>>         rt_mutex_release(&mutex);
>>>         if (err) {
>>>                 printf("Error: rt_cond_signal failed, err = %d", err);
>>>                 exit(-1);
>>>         }
>>>         printf("%s --> cond signal success: %d\n", __func__, err);
>>>         printf("Waiting for task to finish....\n");
>>>         rt_task_join(&task1);
>>>
>>>         return 0;
>>> }
>>> -----------------------------------------------------
>>>
>>> Is there any fix available in Xenomai-3 for this?
>>> Please let me know if there is something wrong here.
>>>
>>>
>>
>> You are not using condition variables the right way, those have no
>> semaphore semantics. Please read a POSIX documentation about them like
>> [1], the alchemy API does not differ in behavior.
>>
> 
> Thank you for your reply.
> Can you point out the mistake in this program?

No, you need to figure this out by yourself, reading a proper
documentation about using POSIX synchronization features. I know, I'm mean.

> I will try to develop the posix equivalent and check.
> I even tried removing mutex from rt_cond_signal, but it did not help.
> 

No, removing what makes condvars working properly is definitely not
going to help.

Please grab a POSIX documentation somewhere on the net, a couple of
illustrations and use cases, then get your feet wet with mutexes and
condition variables: those are the fundamental features POSIX provides
for thread synchronization. There is no escape.

-- 
Philippe.