Signals and semaphores


 
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# 1  
Old 09-02-2010
Signals and semaphores

I have problem with my application. Application is running on embedded Linux machine. It's basically multiprotocol gateway that connects two industrial Ethernet networks.

We are experiencing some kind of application hang every 2 to 3 days. It seems like both threads are still running but SIGALARM signal is getting lost. I'm not completely sure if this is a case because project is in testing phase and I can't make any application changes until end of a week.SIGALARM is used for counter implementation and it is activated every second.

I have semaphores inside signal handler and inside thread. I have initialized SIGALRM with sigaction and SA_RESTART flag. I suspect this is a reason why we are experiencing counter (or application) hang.

Code:
  void start_timer(void)
    {
       struct sigaction sa;
       struct itimerval timer;

       memset(&sa, 0, sizeof (sa));
 
       sa.sa_handler = &timer_handler;
       sa.sa_flags = SA_RESTART;  
       sigaction(SIGALRM, &sa, NULL);

       timer.it_value.tv_sec = 1;
       timer.it_value.tv_usec = 0;
 
       timer.it_interval.tv_sec = 1;
       timer.it_interval.tv_usec = 0;

       setitimer(ITIMER_REAL, &timer, NULL);
    }


    int main(int  argc, char *argv[])
    {
        //  write log entry
        //  check arguments to main
        //  read files containing parameters  of remote devices 
    
        //  initialise semaphores 
        sem_init(&semCounters, 0, 1);
        sem_init(&semPackets, 0, 1);
        sem_init(&semEvents, 0, 1);
        
        start_timer();
        pthread_create(&thread1, NULL, firstThread, NULL);
        pthread_create(&thread2, NULL, secondThread, NULL);
    
        while(1)
        {            
            //  infinite loop
        }
    
        return 0;
    }

Inside a second thread (secondThread) I have multiple math operation with time counters. All counter are protected with semaphores (semCounter). I'm using same counters inside signal handler! Semaphores are in both locations. Below is an example of use reinitialization of protected variable. This code can be found both in signal handler and in thread.

Code:
sem_wait(&semCounters);
    counter.t4 = 0;
    sem_post(&semCounters);

One more thing. I'm creating other threads inside signal handler but I'm not waiting thread to finish processing. Application has been restarted 4 times so far. Longest running time without hang is three days.
# 2  
Old 09-02-2010
First, you haven't posted your signal handler code.

But I don't think sem_wait() is async-signal safe. Which means you can't safely use it in a signal handler. Period. You can't use it.

Which is probably why your app is deadlocking.
# 3  
Old 09-02-2010
Exactly, please post the timer_handler(int signalType) function, so we can see what's wrong.
Zykl0n-B
# 4  
Old 09-02-2010
Quote:
Originally Posted by achenle
I don't think sem_wait() is async-signal safe.
Correct. The only IPC that is, is sem_post. You might be able to create a more signal-safe arrangement wherein the interrupt handler only posts and the thread only waits.

Also, it's a good idea to check the return value of these things. In your current implementation sem_wait and sem_post are probably failing and returning EINTR at least sometimes, which will cause your semaphore to be off by one and lock the program solid.
# 5  
Old 09-03-2010
Quote:
Originally Posted by achenle
First, you haven't posted your signal handler code.

But I don't think sem_wait() is async-signal safe. Which means you can't safely use it in a signal handler. Period. You can't use it.

Which is probably why your app is deadlocking.
You are right about sem_wait(). I didn't know that. I was sure that sem_post() is async-signal safe and that is true. I didn't check for sem_wait().
Below is only a part of my timer_handler function.

Code:
void timer_handler(int signum)                                                  
{                                                                               
                                                                                
    pthread_t updateThread;                                                     
                                                                                
    sem_wait(&semCounters);                                                     
                                                                                
    counter.t1++;                                                               
    counter.t2++;                                                               
    counter.t3++;                                                               
    counter.t4++;                                                               
                                                                                
    if(counter.t4 >= 30 && flag == 1)                                           
    {                                                                           
        if(ret == 1)                                                            
        {                                                                       
            if(DEBUG)                                                           
            {                                                                   
                printf("GPRS General Interrogation RETRY flag set.\n");         
            }                                                                   
                                                                                
            counter.t4 = 0;                                                     
                                                                                
            pthread_create(&updateThread, NULL, thread3Function, NULL);         
        }                                                                       
                                                                                
        else                                                                    
        {                                                                       
            counter.t4 = 0;                                                     
        }                                                                       
    }                                                                           
    sem_post(&semCounters);                                                     
}



---------- Post updated at 02:12 AM ---------- Previous update was at 02:04 AM ----------

Quote:
Originally Posted by Corona688
Correct. The only IPC that is, is sem_post. You might be able to create a more signal-safe arrangement wherein the interrupt handler only posts and the thread only waits.

Also, it's a good idea to check the return value of these things. In your current implementation sem_wait and sem_post are probably failing and returning EINTR at least sometimes, which will cause your semaphore to be off by one and lock the program solid.
You say I can only sem_post() inside a signal handler and only sem_wait() inside a threads? I have semaphores implemented on many places inside thread. What if one of my variable is changed more than once in a second ? - my signal handler goes on every second. Thread will have to wait after one variable change for a signal handler to release semaphore to continue processing.


I'm also not sure if I need to check return value from sem_wait() and sem_post(). I have set SA_RESTART flag so if function call is interrupted by signal, function will be restarted after signal handler is finished.

Last edited by _thomas; 09-03-2010 at 07:14 AM..
# 6  
Old 09-03-2010
You can't call pthread_create() in a signal handler, either.
This User Gave Thanks to achenle For This Post:
# 7  
Old 09-03-2010
I'll try to remove all sem_wait and pthread_create functions from my signal handler, thanks.
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