c - Implementing a FIFO mutex in pthreads

Question

I'm trying to implement a binary tree supporting concurrent insertions (which could occur even between nodes), but without having to allocate a global lock or a separate mutex or mutexes for each node. Rather, the quantity of such locks allocated should be on the order of the quantity of threads using the tree.

Consequently, I end up with a type of lock convoy problem. Explained more simply, it's what potentially happens when two or more threads do the following:

1 for(;;) {
2   lock(mutex)
3   do_stuff
4   unlock(mutex)
5 }

That is, if Thread#1 executes instructions 4->5->1->2 all in one "cpu burst" then Thread#2 gets starved from execution.

On the other hand, if there was a FIFO-type locking option for mutexes in pthreads, then such a problem could be avoided. So, is there a way to implement FIFO-type mutex locking in pthreads? Can altering thread priorities accomplish this?

Answer 1

You can implement a fair queuing system where each thread is added to a queue when it blocks, and the first thread on the queue always gets the resource when it becomes available. Such a "fair" ticket lock built on pthreads primitives might look like this:

#include <pthread.h>

typedef struct ticket_lock {
    pthread_cond_t cond;
    pthread_mutex_t mutex;
    unsigned long queue_head, queue_tail;
} ticket_lock_t;

#define TICKET_LOCK_INITIALIZER { PTHREAD_COND_INITIALIZER, PTHREAD_MUTEX_INITIALIZER }

void ticket_lock(ticket_lock_t *ticket)
{
    unsigned long queue_me;

    pthread_mutex_lock(&ticket->mutex);
    queue_me = ticket->queue_tail++;
    while (queue_me != ticket->queue_head)
    {
        pthread_cond_wait(&ticket->cond, &ticket->mutex);
    }
    pthread_mutex_unlock(&ticket->mutex);
}

void ticket_unlock(ticket_lock_t *ticket)
{
    pthread_mutex_lock(&ticket->mutex);
    ticket->queue_head++;
    pthread_cond_broadcast(&ticket->cond);
    pthread_mutex_unlock(&ticket->mutex);
}