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Date: Wed, 14 May 2008 21:50:29 +0300
From: Andriy Gapon <avg@icyb.net.ua>
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Subject: Re: thread scheduling at mutex unlock
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on 14/05/2008 18:17 Andriy Gapon said the following:
> I am trying the small attached program on FreeBSD 6.3 (amd64,
> SCHED_4BSD) and 7-STABLE (i386, SCHED_ULE), both with libthr as threads
> library and on both it produces "BROKEN" message.
> 
> I compile this program as follows:
> cc sched_test.c -o sched_test -pthread
> 
> I believe that the behavior I observe is broken because: if thread #1
> releases a mutex and then tries to re-acquire it while thread #2 was
> already blocked waiting on that mutex, then thread #1 should be "queued"
> after thread #2 in mutex waiter's list.
> 
> Is there any option (thread scheduler, etc) that I could try to achieve
> "good" behavior?
> 
> P.S. I understand that all this is subject to (thread) scheduler policy,
> but I think that what I expect is more reasonable, at least it is more
> reasonable for my application.
> 
> 

Daniel Eischen has just kindly notified me that the code (as an 
attachment) didn't make it to the list, so here it is inline.


#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>


pthread_mutex_t mutex;
int count = 0;

static void * thrfunc(void * arg)
{
     while (1) {
         pthread_mutex_lock(&mutex);

         count++;
         if (count > 10) {
             fprintf(stderr, "you have a BROKEN thread scheduler!!!\n");
             exit(1);
         }

         sleep(1);

         pthread_mutex_unlock(&mutex);
     }
}

int main(void)
{
     pthread_t thr;

#if 0
     pthread_mutexattr_t attr;
     pthread_mutexattr_init(&attr);
     pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE_NP);
     pthread_mutex_init(&mutex, &attr);
#else
     pthread_mutex_init(&mutex, NULL);
#endif

     pthread_create(&thr, NULL, thrfunc, NULL);

     sleep(2);

     pthread_mutex_lock(&mutex);
     count = 0;
     printf("you have good thread scheduler\n");
     pthread_mutex_unlock(&mutex);

     return 0;
}

-- 
Andriy Gapon