3.5.1 Regulating the Philosophers With Tokens

The following listing shows the fixed version of the dining philosophers program that uses the token system. This solution incorporates four tokens, one less than the number of diners, so no more than four philosophers can attempt to eat at the same time. This version of the program is called din_philo_fix1.c:

    1	#include <pthread.h>
     2	#include <stdio.h>
     3	#include <unistd.h>
     4	#include <stdlib.h>
     5	#include <errno.h>
     6	#include <assert.h>
     7	
     8	#define PHILOS 5
     9	#define DELAY 5000
    10	#define FOOD 50
    11	
    12	void *philosopher (void *id);
    13	void grab_chopstick (int,
    14	                     int,
    15	                     char *);
    16	void down_chopsticks (int,
    17	                      int);
    18	int food_on_table ();
    19	int get_token ();
    20	void return_token ();
    21	
    22	pthread_mutex_t chopstick[PHILOS];
    23	pthread_t philo[PHILOS];
    24	pthread_mutex_t food_lock;
    25	pthread_mutex_t num_can_eat_lock;
    26	int sleep_seconds = 0;
    27	uint32_t num_can_eat = PHILOS - 1;
    28	
    29	
    30	int
    31	main (int argn,
    32	      char **argv)
    33	{
    34	    int i;
    35	
    36	    pthread_mutex_init (&food_lock, NULL);
    37	    pthread_mutex_init (&num_can_eat_lock, NULL);
    38	    for (i = 0; i < PHILOS; i++)
    39	        pthread_mutex_init (&chopstick[i], NULL);
    40	    for (i = 0; i < PHILOS; i++)
    41	        pthread_create (&philo[i], NULL, philosopher, (void *)i);
    42	    for (i = 0; i < PHILOS; i++)
    43	        pthread_join (philo[i], NULL);
    44	    return 0;
    45	}
    46	
    47	void *
    48	philosopher (void *num)
    49	{
    50	    int id;
    51	    int i, left_chopstick, right_chopstick, f;
    52	
    53	    id = (int)num;
    54	    printf ("Philosopher %d is done thinking and now ready to eat.\n", id);
    55	    right_chopstick = id;
    56	    left_chopstick = id + 1;
    57	
    58	    /* Wrap around the chopsticks. */
    59	    if (left_chopstick == PHILOS)
    60	        left_chopstick = 0;
    61	
    62	    while (f = food_on_table ()) {
    63	        get_token ();
    64	
    65	        grab_chopstick (id, right_chopstick, "right ");
    66	        grab_chopstick (id, left_chopstick, "left");
    67	
    68	        printf ("Philosopher %d: eating.\n", id);
    69	        usleep (DELAY * (FOOD - f + 1));
    70	        down_chopsticks (left_chopstick, right_chopstick);
    71	
    72	        return_token ();
    73	    }
    74	
    75	    printf ("Philosopher %d is done eating.\n", id);
    76	    return (NULL);
    77	}
    78	
    79	int
    80	food_on_table ()
    81	{
    82	    static int food = FOOD;
    83	    int myfood;
    84	
    85	    pthread_mutex_lock (&food_lock);
    86	    if (food > 0) {
    87	        food--;
    88	    }
    89	    myfood = food;
    90	    pthread_mutex_unlock (&food_lock);
    91	    return myfood;
    92	}
    93	
    94	void
    95	grab_chopstick (int phil,
    96	                int c,
    97	                char *hand)
    98	{
    99	    pthread_mutex_lock (&chopstick[c]);
   100	    printf ("Philosopher %d: got %s chopstick %d\n", phil, hand, c);
   101	}
   102	
   103	void
   104	down_chopsticks (int c1,
   105	                 int c2)
   106	{
   107	    pthread_mutex_unlock (&chopstick[c1]);
   108	    pthread_mutex_unlock (&chopstick[c2]);
   109	}
   110	
   111	
   112	int
   113	get_token ()
   114	{
   115	    int successful = 0;
   116	
   117	    while (!successful) {
   118	        pthread_mutex_lock (&num_can_eat_lock);
   119	        if (num_can_eat > 0) {
   120	            num_can_eat--;
   121	            successful = 1;
   122	        }
   123	        else {
   124	            successful = 0;
   125	        }
   126	        pthread_mutex_unlock (&num_can_eat_lock);
   127	    }
   128	}
   129	
   130	void
   131	return_token ()
   132	{
   133	    pthread_mutex_lock (&num_can_eat_lock);
   134	    num_can_eat++;
   135	    pthread_mutex_unlock (&num_can_eat_lock);
   136	}

Try compiling and running this fixed version of the dining philosophers program and running it several times. The system of tokens limits the number of diners attempting to use the chopsticks and thus avoids actual and potential deadlocks.

3.5.1.1 A False-Positive Report

In spite of using the system of tokens, the Thread Analyzer reports a potential deadlock for this implementation even though none exists. This is a false positive. Consider the following screen-shot which details the potential deadlock:

Select the first thread in the chain (Thread #2) and then click on the Dual Source tab to see the source code location in which Thread #2 held the lock at address 0x215a8, and where in the source code it requested the lock at address 0x215c0. The following screen-shot shows the Dual Source tab for Thread #2.

The get_token() function in din_philo_fix1.c uses a while loop to synchronize the threads. A thread will not leave the while loop until it successfully gets a token (this occurs when num_can_eat is greater than zero). The while loop limits the number of simultaneous diners to four. However, the synchronization implemented by the while loop is not recognized by the Thread Analyzer. It assumes that all five philosophers attempt to grab the chopsticks and eat concurrently, so it reports a potential deadlock. The following section details how to limit the number of simultaneous diners by using synchronizations which the Thread Analyzer recognizes.