Mutex Lock Code Examples

Here are some code fragments showing mutex locking.

Example 4-1 Mutex Lock Example

#include <pthread.h>

pthread_mutex_t count_mutex;
long long count;

void
increment_count()
{
	    pthread_mutex_lock(&count_mutex);
    count = count + 1;
	    pthread_mutex_unlock(&count_mutex);
}

long long
get_count()
{
    long long c;
    
    pthread_mutex_lock(&count_mutex);
	    c = count;
    pthread_mutex_unlock(&count_mutex);
	    return (c);
}

The two functions in Example 4-1 use the mutex lock for different purposes. The increment_count() function uses the mutex lock simply to ensure an atomic update of the shared variable. The get_count() function uses the mutex lock to guarantee that the 64-bit quantity count is read atomically. On a 32-bit architecture, a long long is really two 32-bit quantities.

Reading an integer value is an atomic operation because integer is the common word size on most machines.

Using Locking Hierarchies

You will occasionally want to access two resources at once. Perhaps you are using one of the resources, and then discover that the other resource is needed as well. There could be a problem if two threads attempt to claim both resources but lock the associated mutexes in different orders. For example, if the two threads lock mutexes 1 and 2 respectively, then a deadlock occurs when each attempts to lock the other mutex.

Example 4-2 Deadlock

The best way to avoid this problem is to make sure that whenever threads lock multiple mutexes, they do so in the same order. This technique is known as lock hierarchies: order the mutexes by logically assigning numbers to them.

Also, honor the restriction that you cannot take a mutex that is assigned n when you are holding any mutex assigned a number greater than n.

The lock_lint tool can detect the sort of deadlock problem shown in this example. The best way to avoid such deadlock problems is to use lock hierarchies. When locks are always taken in a prescribed order, deadlock should not occur.

However, this technique cannot always be used--sometimes you must take the mutexes in an order other than prescribed. To prevent deadlock in such a situation, use pthread_mutex_trylock(). One thread must release its mutexes when it discovers that deadlock would otherwise be inevitable.

Example 4-3 Conditional Locking

In this example, thread 1 locks mutexes in the prescribed order, but thread 2 takes them out of order. To make certain that there is no deadlock, thread 2 has to take mutex 1 very carefully; if it were to block waiting for the mutex to be released, it is likely to have just entered into a deadlock with thread 1.

To ensure this does not happen, thread 2 calls pthread_mutex_trylock(), which takes the mutex if it is available. If it is not, thread 2 returns immediately, reporting failure. At this point, thread 2 must release mutex 2, so that thread 1 can lock it, and then release both mutex 1 and mutex 2.

Nested Locking with a Singly Linked List

Example 4-4 and Example 4-5 show how to take three locks at once, but prevent deadlock by taking the locks in a prescribed order.

Example 4-4 Singly Linked List Structure

typedef struct node1 {
    int value;
    struct node1 *link;
    pthread_mutex_t lock;
} node1_t;

node1_t ListHead;

This example uses a singly-linked list structure with each node containing a mutex. To remove a node from the list, first search the list starting at ListHead (which itself is never removed) until the desired node is found.

To protect this search from the effects of concurrent deletions, lock each node before any of its contents are accessed. Because all searches start at ListHead, there is never a deadlock because the locks are always taken in list order.

When the desired node is found, lock both the node and its predecessor since the change involves both nodes. Because the predecessor's lock is always taken first, you are again protected from deadlock. Example 4-5 shows the C code to remove an item from a singly linked list.

Example 4-5 Singly-Linked List with Nested Locking

node1_t *delete(int value)
{
    node1_t *prev, *current;

    prev = &ListHead;
    pthread_mutex_lock(&prev->lock);
    while ((current = prev->link) != NULL) {
        pthread_mutex_lock(&current->lock);
        if (current->value == value) {
            prev->link = current->link;
            pthread_mutex_unlock(&current->lock);
            pthread_mutex_unlock(&prev->lock);
            current->link = NULL;
            return(current);
        }
        pthread_mutex_unlock(&prev->lock);
        prev = current;
    }
    pthread_mutex_unlock(&prev->lock);
    return(NULL);
}

Nested Locking with a Circular Linked List

Example 4-6 modifies the previous list structure by converting it into a circular list. There is no longer a distinguished head node; now a thread might be associated with a particular node and might perform operations on that node and its neighbor. Note that lock hierarchies do not work easily here because the obvious hierarchy (following the links) is circular.

Example 4-6 Circular Linked List Structure

typedef struct node2 {
    int value;
    struct node2 *link;
    pthread_mutex_t lock;
} node2_t;

Here is the C code that acquires the locks on two nodes and performs an operation involving both of them.

Example 4-7 Circular Linked List with Nested Locking

void Hit Neighbor(node2_t *me) {
    while (1) {
        pthread_mutex_lock(&me->lock);
        if (pthread_mutex_lock(&me->link->lock)!= 0) {
            /* failed to get lock */             
            pthread_mutex_unlock(&me->lock);              
            continue;         
        }         
        break;     
    }     
    me->link->value += me->value;     
    me->value /=2;     
    pthread_mutex_unlock(&me->link->lock);     
    pthread_mutex_unlock(&me->lock);
}