pthread_mutex problem

rotis23 · 11-16-2005, 09:27 AM

Hi All,

Consider this [multi-threaded] code fragment...

Code:

int volatile num_threads = 0;

snip...

if(num__threads < MAX_NUM_THREADS)
{
	//mutex
	pthread_mutex_lock(&my_thread_mutex);
	//increment number of used threads
	num_threads++;
	pthread_mutex_unlock(&my_thread_mutex);
	//mutex

	printf("%d:DEBUG: num threads = %d\n",pthread_self(),num_threads);

	//do work
	if(do_work() < 0)
	{
		//mutex
		pthread_mutex_lock(&my_thread_mutex);
		//decrement number of slaves
		num_threads--;
		pthread_mutex_unlock(&my_thread_mutex);
		//mutex

		return 1;
	}

	//mutex
	pthread_mutex_lock(&my_thread_mutex);
	//decrement number of slaves
	num_threads--;
	pthread_mutex_unlock(&my_thread_mutex);
	//mutex

	return 0;
}

I'm trying to throttle the number of pthreads accessing this bit of code. The mutexes are initialised and destroyed elsewhere.

The problem is that num_threads ends up negative i..e somehow the decrement is happening more than the increment.

Can anyone see why?

Are my mutexes used correctly?

Thanks for any help,

rotis23

IfYouSaySo · 11-16-2005, 12:53 PM

It looks fine. Is num__threads being modified anywhere else? By the way, I recommend the following small change:

Code:

int volatile num_threads = 0;

snip...

if(num__threads < MAX_NUM_THREADS)
{
        int result;
	//mutex
	pthread_mutex_lock(&my_thread_mutex);
	//increment number of used threads
	num_threads++;
	pthread_mutex_unlock(&my_thread_mutex);
	//mutex

	printf("%d:DEBUG: num threads = %d\n",pthread_self(),num_threads);

	//do work
        result = do_work();

	//mutex
	pthread_mutex_lock(&my_thread_mutex);
	//decrement number of slaves
	num_threads--;
	pthread_mutex_unlock(&my_thread_mutex);
	//mutex

        if (result < 0)
            return 1;
        else
            return 0;
}

rotis23 · 11-17-2005, 05:42 AM

Thanks for the input.

num__threads should be num_threads in my code fragment.

num_threads does not change anywhere else.

Was thinking of doing that change - much more readable

BTW - this is running on an *old* redhat 8.0 platform: any bugs on that platform that would cause this? <clutching at straws>

rotis23 · 11-17-2005, 11:55 AM

Something very weird going on here...I've seen the debug printf line output MAX_NUM_THREADS+1 !

How is that possible? Either I've implemented the mutexes wrong or they're not working properly.

Jaqui · 11-17-2005, 05:09 PM

RH8 [ all distros with the 2.4 kernel actually ] had a few small problems with libpthread, that were fixed in the 2.6 kernel.

you my be hitting one of the known issues of the 2.4 kernel with pthreads.

IfYouSaySo · 11-21-2005, 12:51 AM

I doubt if there were issues with the mutexes in Redhat 8. Maybe other areas, but I bet the mutexes were fairly solid. Anyway, I can see why you would end up with MAX_THREADS+1. Look at the code. You are doing your conditional check on the num_threads outside of the mutex. Do you see why that is a problem? Imagine that you are sitting at num_threads = MAX_THREADS-1 currently. Then thread1 comes along. It says, num_threads less than max, I can continue. Then context switch happens, and control goes to thread2. It looks at num_threads, says num_threads is less than max, I can continue. So then each thread obtains the mutex and increments num_threads. Now you have num_threads == MAX_THREADS+1. So change the logic to something like:

Code:

    pthread_mutex_lock(&mutex);

    if (num_threads > MAX) {
        num_threads++;

        // do work
    }
    else {
        pthread_mutex_unlock(&mutex);
    }

rotis23 · 11-23-2005, 08:01 AM

Yes thanks - I can see MAX_THREADS+1 but not num_threads < 0.

bithub · 11-24-2005, 12:15 PM

Is that your real code? I don't see how that code alone can generate a num_threads value less than zero.

rotis23 · 11-25-2005, 09:00 AM

I think this might have been a memory violation issue. I was attempting to access a NULL struct later in the thread and I'm guessing this could have potentially violated memory?

bithub · 11-25-2005, 11:50 AM

Anytime you attempt to dereference invalid data, strange things can (and usually do) happen in your program.

11-17-2005, 05:42 AM	#3
rotis23 Registered User Join Date: Aug 2002 Posts: 351	Thanks for the input. num__threads should be num_threads in my code fragment. num_threads does not change anywhere else. Was thinking of doing that change - much more readable BTW - this is running on an old redhat 8.0 platform: any bugs on that platform that would cause this? <clutching at straws>
rotis23 is offline

11-17-2005, 11:55 AM	#4
rotis23 Registered User Join Date: Aug 2002 Posts: 351	Something very weird going on here...I've seen the debug printf line output MAX_NUM_THREADS+1 ! How is that possible? Either I've implemented the mutexes wrong or they're not working properly.
rotis23 is offline

11-21-2005, 12:51 AM	#6
IfYouSaySo Registered User Join Date: Aug 2002 Location: Hermosa Beach, CA Posts: 446	I doubt if there were issues with the mutexes in Redhat 8. Maybe other areas, but I bet the mutexes were fairly solid. Anyway, I can see why you would end up with MAX_THREADS+1. Look at the code. You are doing your conditional check on the num_threads outside of the mutex. Do you see why that is a problem? Imagine that you are sitting at num_threads = MAX_THREADS-1 currently. Then thread1 comes along. It says, num_threads less than max, I can continue. Then context switch happens, and control goes to thread2. It looks at num_threads, says num_threads is less than max, I can continue. So then each thread obtains the mutex and increments num_threads. Now you have num_threads == MAX_THREADS+1. So change the logic to something like: Code: pthread_mutex_lock(&mutex); if (num_threads > MAX) { num_threads++; // do work } else { pthread_mutex_unlock(&mutex); } __________________ The crows maintain that a single crow could destroy the heavens. Doubtless this is so. But it proves nothing against the heavens, for the heavens signify simply: the impossibility of crows.
IfYouSaySo is offline

11-23-2005, 08:01 AM	#7
rotis23 Registered User Join Date: Aug 2002 Posts: 351	Yes thanks - I can see MAX_THREADS+1 but not num_threads < 0.
rotis23 is offline

11-24-2005, 12:15 PM	#8
bithub Registered User Join Date: Sep 2004 Location: California Posts: 3,020	Is that your real code? I don't see how that code alone can generate a num_threads value less than zero.
bithub is offline

11-25-2005, 09:00 AM	#9
rotis23 Registered User Join Date: Aug 2002 Posts: 351	I think this might have been a memory violation issue. I was attempting to access a NULL struct later in the thread and I'm guessing this could have potentially violated memory?
rotis23 is offline

11-25-2005, 11:50 AM	#10
bithub Registered User Join Date: Sep 2004 Location: California Posts: 3,020	Anytime you attempt to dereference invalid data, strange things can (and usually do) happen in your program.
bithub is offline

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